Black Magic have released new drivers for their range of capture cards which also include support for Windows 7.  You must use the latest Premiere update with them (4.2) as well. 
We have not yet tested these drivers with either Premiere 4.2 or Windows 7 but from other reports they are working well.
Adobe have already said that CS4 works fine on Windows 7 (something with which we can concur based on experience).