Any reason the Vista x64 drivers wouldn't work in Win7? For the Theater 650 Pro for example, they've got a driver dated just in the last week.
ATI Continues its downsizing of Tuners
They do work (I've used) but as Mike mentioned, I'd be very weary of buying a new ATI Tuner. ATI actually did something very similar around Vista RTM.
The ATI Theater 650 Pro at initial released said Windows Vista compatible on the box but they did not release the drivers for almost 9 months. During the AMD\ATI merger they also dropped the All-In-Wonder cards, no driver development at all and then brought them back a year or so later.
FWIW, yes, the Vista drivers for the 650 work in Windows 7. The installation initially fails, but then you can go into device manager and set up the drivers.
That said I don't have the latest and greatest drivers if they were just updated, so YMMV.