There have been a few rumours floating around that T-mobile won't have it on the 24th though. I won't be happy if they don't, but it just means I'll switch to Orange as I'm at the end of contract.
Lots of people have been asking in stores about micro sim cards (check the UK iphone 4 thread) and for the most part it seems that T-mobile staff don't even know what one is, so I think that they're gonna be hard pushed to get everything rolled out for launch day.
I really hope they do, but don't hold your breath!