So I'm still just getting it set up, but I have Windows 7 (beta) 64-bit running on a Boot Camp partition of a MacBook Pro (13" new uni-body) with OS X 10.5.3 and WSJT works just fine.
Only mild headache is the 13" Macbook comes with a headphone/mic port that is shared so I did an end-run and connected my Griffin iMic (which Windows 7 recognized right away and pulled the driver down automagically from the vendor's site) and that way I could still listen to a podcast on the laptop speakers while WSJT has exclusive use of my iMic soundcard.
To convert the COM port on my BUXCOMM RASCAL (because my other interfaces don't travel as easily) I have a KEYSPAN USA-19HS COM->USB adapter.
I'm no huge fan of Microsoft, but a lot of the issues I had run into when running/supporting VISTA (all flavors) with a lot of ham software seem to be alleviated mostly by the "XP Compatibility" mode you can run applications within.
All this is connected to my Yaesu FT-857D (whether it's for HF JT65A or m/s).
That said, I'd sure celebrate the day I can successfully compile WSJT on OS-X. :)
73 and Good DX!