It’ll be a party for me because it’ll be my 28th birthday. (I happen to be born on September 11th). 
But I think it’s fantastic that they want to rebuild the WTC. All power to everyone involved.
Oh, I wouldn’t be so hasty. While there will be young children who weren’t alive when it happened, the majority of the living population will still remember it, and quite vividly, I believe.
Granted, in decades to come, it will become a day when the History Channel airs several hours of documentaries about 9/11/01, just like they show Pearl Harbor (heh I almost typed “Pearl Jam”) documentaries on 12/7, and JFK doc’s on 11/25. Heck, we hear a lot of Elvis on the radio on 8/16, and he’s been dead for 26 years.
Personally, I love the new WTC design. Especially how the tall tower will top out at 1,776 feet, thus beating those Malaysian upstarts in the “skyscraper race”, and shutting up those pesky Canadians as well. 
KGS, I agree that people will continue to remember September 11. It’s helped in that regard by the fact that it’s referred to almost uniquely by its date. In US History, only July 4 even comes close to that (“Independence Day” is an equally-used [IMHO] synonym). Some people may say “WTC and Pentagon attacks”, but the date is the sole reference point for most people around the world.
However, unless the September 11, 2001 attacks become the crucial starting point of a much bigger picture (i.e. turn out to be for a future WWIII what Pearl Harbor was for WWII [from a US perspective]), I really think that remembrance of the date will fade over the years. I am a baby-boomer, and grew up very aware of the important dates of WWII, yet have been surprised at the extent to which those dates have faded from common knowledge even during my lifetime. Pearl Harbor (Dec 7, 1941) gets some reprieve because of FDR’s “date which will live in infamy” speech, but the number of people who can identify the dates of VE day, VJ day, or even D-Day in Normandy dwindles each year. How many US non-historians remember important dates in WWI? Or the American Civil War?
I wouldn’t make the following nitpick if it weren’t pertinent:
JFK was assassinated on November 22, 1963. So, less than 30 years later, it’s become a date that it’s perfectly excusable to forget. That’s not how Americans felt in the 1960’s, AFAIK. It’s acceptable nowadays not to know the date of JFK’s death. (That isn’t intended as a judgement call, just an observation.)
So, back to the OP. Yes, People will remember September 11, 2001 - if only because the date is the unique identifier (a lot of people know that JFK was assassinated, but don’t remember the date). The unveiling of the WTC replacements will be a Media Event in the US, partly because NYC is the “Media Capital of the World”. The rest of the world in 2012, however, will glance at the brief coverage in their morning paper and move on.
Sorry, but that’s what I really think.
That should, of course, read “So, less than 40 years later, it’s become a date that is perfectly excusable to forget.”.
No way can I forget that date–it’s my parents’ wedding anniversary! Before 1963 I could never remember their anniversary. After 1963 there was a handy reminder every year (“Next Sunday is the eighteenth anniversary of the death of President Kennedy…”). But that petered out in the 80s.