printing from iPad via Airport Express
The short answer is yes, with limitations.
I connected my USB printer to my Airport Express and then shared the printer on my MacBook. Several apps on the iPad enable printing to a shared printer on the network. I used AirSharing with good results. The limitation is you have to open your document in AirSharing (or whatever app you use) to print it.
If you seriously want to print from your iPad, I strongly suggest buying a new HP printer with AirSharing. I bought a new color LaserJet for only $230 and it is amazingly simple to print with the built-in print functions on the iPad!