My sweetheart (Hi, Rick!) is coming from England to see me here in Detroit next week. We would like to go across the river to Windsor, Canada for the day, but I have heard horror stories about non-US citizens not being readmitted to the US. These stories sound like urban legends to me, but they have me a bit worried. Does anyone know the straight dope?
If he has his passport (and visa if necessary, but I don’t think Brits need one for brief visits) in order, there should be no more problem than you’d have getting into the country in the first place. Especially at a busy border crossing like Windsor-Detroit. If you’re not breaking any laws, they’re more than happy to let you in and spend more of your money. I hate to say it, but it also helps if he’s white, well-dressed and -coiffed (or at least two of the three).
The American side does tend to be more, shall we say, protective? of who (whom) exactly they let into the U. S. of A. When we went to camp in Ontario for one (1) night, crossing over from Port Huron, and then trying to get back in at Niagara Falls, the American customs guy was just this side of snotty because all we had for ID was our American driver’s licenses.
So as long as the BF has his Brit passport to prove that he DOES have a home, and he’s not trying to sneak in and get onto Uncle Sam’s welfare rolls, he ought to be okay. 
But IANA lawyer, so don’t sue me if they lock him up and throw away the key.
However, this website seems to be saying that he does need a visa, of some kind. Somebody wanna wade through this?
http://www.wright.edu/academics/internation/travel-reentry.html
Why would anyone ever want to spend a day in Windsor?
Some of the ports and points of the north shore of Erie are wonderful, but Windsor? Eeeewwww!
I’d guess for the casino
Tell him to check with the US Embassy or Consulate nearest him. There are some odd rules; for some countries, the tourist visa is issued automatically if and only if the visitor comes to the US directly from his home country. I have no idea if the UK is one of those countries or not.
If you are a visitor on the visa waiver scheme, you will have a card stapled into your passport. This allows you 3 months in the country and allows readmission during that stay. Never tried crossing to Canada, but have regularly crossed to Mexico and back with no problem. DO take the passport though!
Maybe she wants to take him to the >>>cough,cough<<< ‘ballet’ (and if she does, all I can say is WHAT A WOMAN!!!)
Ok, I’ll bite. What’s the “cough, cough ‘ballet’”? I wanna be WHAT A WOMAN!!!
Thanks, everyone, for the responses. 
Duck Duck Goose, you are a treasure. Thanks for the explanation (and for the link above as well).
We will be spending first week of his visit in Las Vegas, so Rick might get his fill of casinos and skin there. On the other hand, after not seeing me for four and a half months, it had better be my skin he is interested in. 
My family’s from Windsor. It’s a good place to be FROM, if you catch my drift.
Do evening sweetheart cruises still run up the river from Amhurstburg?
I don;t know, but I will have to check into it. Thanks for the suggestion, Muffin.