Yep. I've got a Buffalo 802.11g bridge for my Tivo. Nothing really to it, you configure it like an AP basically and then just plug it in and leave it.
I think you get a slowdown even if the 802.11b node isn't transmitting, it just has to be associated with the 802.11g AP. The 802.11g AP has to give the 802.11b node a chance to transmit every so often. You can fiddle with the settings on the 802.11g AP to give more time to the 802.11g nodes however but you will still end up with some performance decrease.
There was an article that did tests on what happened on an 802.11g network once you associated an 802.11b device. I'll see if I can find it again.
[edit]Can't find the original article at the moment but Tom's Hardware have a reasonably good article on
802.11g[/edit]