Well at least one PocketPC IM client can work with Google Talk. PocketPC thoughts has a post about it here. The client they mention is here. Since it is Jabber based I'm sure there are others.
Now if only they get the VOIP stuff working from mobile devices...
Google Talk get's their audio technology from Global IP Sound. If you take a look at the data sheet for their voice engine you'll see that it already supports Windows Mobile and Symbian. So I'm hoping we'll see this soon.