I thought of this last night, and then noticed today there was a ‘Africa vanished’ thread. No connection, unless I saw it subconsciously before.
I’m mostly interested in the political and economic effects, so assume for purposes of argument that the 50 states and all their inhabitants vanish and are instantly replaced by salt water. Sea levels remain more or less the same. No more Great Satan. American citizens abroad survive.
No more American Culture to be disseminated. The rest of the world would have to come up with their own versions of Michael Jackson and Heidi Fleiss and Chicken Dance Elmo.
I’d really worry about where all my stuff went.
Marc
Iraq would breathe a sigh of relief and then proceed to exterminate the rest of the world withe their WMD. . . or maybe not.
Well, for starters, Taiwan would be absorbed by China in the time it takes me to write this sentence. Israel would probably take a much more hardline stance against the Palestinians. Basically, any nation that exercised restraint as a result of US influence would say “screw it” and do whatever it is they wanted to do. I think there would be at least short-term instability in many parts of the world, particularly the middle-east, and not instability of a good sort. All of the nations that currently have a flippant view towards civil rights - Cuba, China, etc. - would escalate the extent of their police states.
The global economy would tank, of course. The sudden removal of an entity as huge and influential as the US from the world market would be disastrous. Imagine the US economy if all of the Fortune 500 countries suddenly disappeared.
I’m tempted to say that N. Korea may form an alliance with China and take over S. Korea; I could see how it would be in both of their best interests. However, China’s reluctance to see another (quasi-)world power so close to home may keep that from happening.
Oh, and thousands of US soldiers world-wide would be displeased to find themselves suddenly unemployed.
Jeff
The SDMB would disappear. 
D’oh! That should read “Fortune 500 companies”, of course.
Jeff
Canada would build up a large military force in order to defend its borders. The left over americans would populate Canada. The Canadian economy recieves a boost in labour and military and its economy grows rapidly. The Canadian governement adjusts itself in order to be more welcoming to the American settlers. The new founded United Canadian Republic becomes the world power. The world is a more polite place.
Millions of Canadians revel in their new beachfront property and fishing rights.
Hmm, if the USA just vanished, why would it have to be replaced with water? Why not a giant, infinitly deep black void? And if water did rush in from everywhere else, wouldn’t that lower sea level worldwide. Also, with the US vanishing, would its coastal waters also disappear, or just the land and inland waters? What about Hawaii?
Perplexing. 
Um, because in the situation presented in the OP, the US is replaced by salt water, that’s why.
African countries would be able to make cheap AIDS drugs.
A lot of Third World countries with a bucketload of debt would suddenly have a lot more spare cash each year
UK arms manufacturers, freed from their biggest competitors, would make out like bandits.
I was thinking along the lines of ElJeffe, but I’d add this: Russia would suddenly find itself back in the superpower slot by default due to its remaining nuclear arsenal. Whether Putin and the boys decided to stick with NATO or assert their new relative power would be a big turning point.
Also, along with worldwide depression, worldwide famine due to the disappearance of the U.S. breadbasket.
Hmm, Canada is larger than Australia, so I guess we’d be considered a “continent”, though we’d become the most common wrong answer to the Trivial Pursuit question: “What is the world’s largest island?”
Almost every other democracy would have to increase its defense spending dramatically. They’d have to scale back social programs to pay for it.
So soon, the whole free world would be like the U.S.A. (-:
That’s the first thing that popped into my head, too 
I doubt China would take over Taiwan. Military analysts are divided on who would win should the two go to war. Despite China’s overwhelming size advantage, Taiwan’s army is much better equipped, more hi-tech, leaner, meaner, and extremely hunkered down. It would be a long drawn out affair.
China would not side with North Korea now as there are no benefits in doing so, and the few remaining Communist idealists in China’s government are outnumbered by the get rich quick brigade.
Religions all over (esp the Vatican) would encrease exponentially in power. There would be lots of Mexicans swimin’ and complaining that they didn’t think the Rio Grande was that wide.
Now hold on a minute here:
“worldwide famine due to the disappearance of the US breadbasket”?
Just who is the US currently feeding for free? I think that without the US enforcing trade treaties designed exclusively to further its own interests, suddenly the developing world would be much better off. The US is also the only real hawk on debt forgiveness for developing countries. Japan and the EU would forgive debts, obviously the US couldn’t be repaid, and suddenly every African government is in much, much healthier financial shape.
“any nation that exercised restraint as a result of US influence would say “screw it” and do whatever it is they wanted to do”
If N. Korea, Israel and China are exercising any restraint at the moment, I’ve not noticed it. Korea has just re-activated its nuclear program, Israel is executing Palestinians at will and China is as expansionist and repressive as ever. Cuba is an exception, I’ll grant you.
I suspect if the US vanished, Israel would probably not survive long. The EU is more supportive of Arabic nations than is the US, and Israel gets credibility – as well as shedloads of cash – from the US.
There would, I am sure be a massive downturn in the world’s economy. Markets hate uncertainty more than anything, and the removal of the world’s economic powerhouse would move the rest of the world into unprecedented new territory.
No more Simpsons! That would be very bad.
snort! Ha! Even if they did have more money, there wouldn’t be any food to buy. North Korea alone would see millions starve. And, you know, the massive worldwide economic crash would wip out any wealth remaining.