We have a sturdy little USB HP LaserJet 1022 printer. We've used it very happily, over Wifi; I connected the USB cable to the Apple Airport Express, and then used the Add Printer option to locate the correct drivers. The printer worked like a charm over WiFi, and was dead easy to set-up.
Flash forward: I have a new MacBook, and Snow Leopard. It would print a single document via Wifi to the HP LaserJet, then stop with the gray flashing exclamation point next to the printer name in System Preferences/Print & Fax. It didn't matter what application I used, or what font. The Log said that my Mac couldn't communicate with the printer. I figured it was using an old driver.
I reset the System Preferences/Fax & Print settings, for a fresh start. (Pretty much after each solution failed, I would Ctrl + click the list of printers in System Preference/Fax and Print and select Reset printing system).
I deleted the drivers. I used Software Update to download the latest drivers. No joy.
I discovered that I'm not alone in the world; lots of people are having the same sorts of problems using an HP LaserJet 1022 over WiFi. And it is a supported printer.
At this point, I'd lost more than half a day to trying to get the HP LaserJet 1022 to work via WiFi.
Last week I connected the USB cable to an iMac, went to System Preferences/Sharing/ on the iMac and turned on Printer Sharing.
The printer works beautifully now from any Mac on our WiFi LAN, but I haven't giving up on getting the printer to work over Wifi via the AirPort Express. I'm still seeing lots of others with the same problem.