I have an old Canon MP500 printer. It predates AirPrint and is USB only. With my MacBook Pro running the Printopia app and my printer connected to it, my iPad and iPhone are able to see my printer. It's easy to set up, and although Printopia costs $19.95, there is a free seven-day trial you can use to make sure it works with your setup.
To download the free trial, head to Ecamm Network's Web site. You'll need to be running Mac OS X 10.5 or later (there is no PC version of the app) and iOS 4.2 or later. Printopia doesn't open as an application but instead opens as a pane in your Mac's System Preferences. You'll find it listed at the bottom of the System Preferences window under Other.
On the Printopia window, an on/off button sits on the left and a list of available printers sits on the right. As long as everything is connected to the same network, Printopia should find your printer automatically, whether a printer is connected via USB to your Mac or via a wired or wireless connection to your Wi-Fi router. Unless you have a Mac Mini and always leave it running, one drawback is your Mac needs to be running and connected to your printer in order to print from and iDevice using Printopia.
I was able to print from my iPad 2's Mail app, Photos app, Web pages in Safari, and iWork apps Pages, Numbers, and Keynote. You can also use your Mac as a virtual printer. Instead of to the printer, you can send it as a PDF to your Mac. Lastly, by clicking on the + button below the printers pane in the Printopia window, you can add another virtual destination, such as a specific folder on your Mac or even Dropbox, or an application such as iPhoto.
If you don't see you printer listed, make sure Printer Sharing is turned on. Go to System Preferences > Print & Scan and click the Sharing Preferences button. On the next window, make sure Printer Sharing is checked. Also, for your chosen printer, make sure to check the box for "Share this printer on the network" on the Print & Scan page.
Although $20 might be too much to spend for the occasional print from your iDevice, if you wield your iPad or iPhone as a business tool, then it might be worth it. You could also try the AirPrint Activator, which asks only for a donation. For Windows users, there's FingerPrint, and you can always try printing from your iPad using Google Docs.