In the last two days, I’ve had the same question come from two different sources. The first time was Friday during an on site iPad workshop I led for 20 educators. The second time was online today as I spent some time addressing questions about the iPad over at iPad4Edu. The question asked was “How can I print from the iPad?”
UPDATE 3/5 Best reviewed app for printing from your iPad was PrintCentral ($9.99). PrintCentral offered the ability to print directly from an iPad to most Wi-Fi or shared printers, but is no longer available in the App Store. However, Print n Share ($8.99) from the same company is found there.
Another option with good reviews is Printopia from ecamm. Installed on Mac as a print server.
Many newer printer models offer the ability to e-mail directly to the printer. HP for example.
More general info on this in NY Times article Printers Adapt to World of the Web.