I have a venerable HP Deskjet 990cxi with a double-sided print feed attachment. It has worked brilliantly through all the versions of Mac OS X I've used. However, with Snow Leopard, I'm stuck with using single-sided printing via the free Gutenberg printer driver, as HP has declined to support this model in Snow Leopard.
After some experimentation, I've found that the native HP DeskJet 9800 series driver seems to work fine -- I've got my double-sided printing back. Additionally, the driver also seems to be giving me ink supply levels details, functionality that the original driver never had.
Hope this is of use to anyone else out there struggling to get this printer working in Snow Leopard.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20091103060917506