The iPad only detects AirPrint printers automatically.
The AirPort Express USB port works quite well with other Apple computers, and even Windows with some minor tweaking, but does not make itself AirPrint ready. Apple may change this in the future, but for now you are stuck with the same options as before.
Apps like Share n Print that can detect and use most wireless printers (this wont' include the Airport Express which works by other means). Must be operating in the background to work.
Apps for specific prints like Canon or HP that have limited print capability, but print directly to the wireless printers they support.
App and computer program combinations that work together to make printers connected to the computer available. Too numerous to begin sorting out.
Programs on the computer that pretend to be AirPrint printers, making printers the computer can see available and sometimes offering extra options. Works best if you have a computer you leave on all the time. Suggestions here are FingerPrint, Printopia, and for free AirPrint Activator (less features than the first two).
http://support.apple.com/kb/HT4356
It's been pointed out that this list is not complete. You'll need to delve into HP's site to get a completely up to date list. I found it once upon a time but forgot to bookmark it.
Update: Ok, I decided not to be quite so lazy. Here is HP's Airprint page. Mouse over the printer pictures to get a list of supported printers. I'm not sure this is the complete list. It is probably just the complete list of the current models.
http://www.hp.com/united-states/campaigns/airprint/about.html
Update 2: A better link.
http://h10025.www1.hp.com/ewfrf/wc/...cc=us&dlc=en&lc=en&jumpid=reg_R1002_USEN#N249