For the Printer Sharing feature of OneCare to work, the printer needs to be locally attached to a host PC, which will then offer to share it to other PCs in the Circle. Vista and XP can share a printer this way as long as Vista is 32 bit. 64 bit Vista can't share a printer to XP or from XP using the OneCare Printer Sharing feature.
Note that this doesn't preclude your sharing the printer normally using Windows File and Printer Sharing functionality. OneCare tries to simplify this process for you, but it won't work for a wireless print server based printer.
-steve