MacWorld reviews 3 printing apps for iPad/iPhone here. The sweet spot seems to be ePrint, an excellent app from Microtech that costs $3.

Jeff Merron also wrote this excellent companion piece about the entire iPad/iPhone printing situation. His final advice: try before you buy. To get the feel of printing from these devices, try a free app first. You may find that e-mailing files works just fine 90% of the time.