Are US PhD admission offers binding? So I have just accepted (via some online function on the website) an offer of PhD admission into a prestigious US university. I have also been in telephonic and email contact with staff at this university. I still feel like it is rather informal. The department told me more information would arrive in April.

What I want to know is, is the offer I have accepted binding on the part of university? I have (and am about to decline) concrete offers from some UK groups and I am worried that I will be burning bridges. Can a US university rescind their offer? Does this ever happen? Am I just being paranoid?

For example, as I am still in master's program, my other offers usually said that I was admitted on the condition that I obtained a sufficiently good grade in my degree etc. My US offer does not make mention of anything like this - not that I plan on doing badly but it is good to know. Would the department think it odd if I asked along these lines?
As far as I know (at least here at Georgia Tech) all the conditions (if there are any) are included in your offer. I have never heard of us rescinding an offer after it has been accepted.  If there is a deadline, though, you must accept by then or the offer may no longer be valid.

That said, I think nobody would be offended if your asked, if it puts your mind at ease.  I would not mention the "getting good grades in your MS" or similar things.

Just send a note to whomever you have been in contact with and say something like "I'm very happy to have been accepted into the program, and am looking forward to coming.  I am pretty sure I've completed everything I need to do to accept admission, but I just wanted to check one last time. I have a number of other acceptances I'm about to decline and I just wanted to check that everything is in order with my admission before I decline the other offers.  Thanks for your help and understanding!"