I had, during iOS 4.2 beta testing, my USB HP CM1017 color laser printer shared via my iMac working on my iOS 4.2 test devices. We've now moved to a new apartment and when setting up our office I did not want to use a USB printer anymore.
We had a couple of unused Airport Express (with USB ports) devices so I figured it would work and was worth a go. Indeed it was!
Setup your Airport Express to join your existing wireless network, you should update the firmware if needed. Restart the device, turn on and connect your printer. Using your Airport Utility make sure it detects your USB printer.
On one of your networked Mac's (I used my 2nd floor EyeTV Mac Mini since it stays on all the time), download and install Airprint Activator LINK. Now just add and setup your "Bonjour" printer and make sure you share it.
WOW, there is no 3! On your iOS 4.2 or newer devices open an app (like Elemental PToE that offers printing), select your printer and off you go!
Here's several thousand words...
Just to clarify, even though all the other Mac's on your network will work with the shared printer on the Airport express, your iOS devices will only "see" Airprint capable printers which is why you need to install Airprint Activator.