I have a Linksys wired/wireless router and the network includes a couple of Windows PCs plus my two Macs. After I got my PB I decided that I wanted to give the iMac and the PB their own printer, just shared between the two of them, so I bought the Airport Express doohicky and set up an HP Photosmart 8400 printer to work wirelessly with only those two machines. I put an older HP printer on the old PC in case I should happen to want to print something on the rare occasion that I'll be using the thing from now on.... and that works out just fine. The older printer isn't shared with the two Macs and the newer printer that the two of them share isn't shared with the other machines (PCs) in the network. Hope this helps! The trick is to set up the Airport Express with just your Macs....
OTB