Since the release of Snow Leopard on Friday, there have been lots of reports of printer incompatibilities, mostly involving Hewlett-Packard and Epson printers. Both companies have responded with fairly detailed compatibility information.
HP has published a compatibility list and instructions for getting supported printers and all-in-ones working under Snow Leopard. No word yet on when drivers for models not currently support might be available.
Epson has also put up a Web page where you can check the compatibility of specific printer and scanner models.