Our good friends at Download Squad found a great app on the App Store called Air Photo that will let you wirelessly print pictures directly from your iPhone or iPod touch. Just install the app (for $1.99, cheap if you print your iPhone's photos often) on your mobile device, put the server on your Mac (or PC, if you swing that way), and then you can snap and print at will. Connecting to the server from your device opens up a Bonjour window, where you can crop or adjust the picture and then print whatever you like.
While the technology itself won't be that helpful for everyone (who really prints pictures any more, especially iPhone pictures?), it is interesting to see this done so easily and well with a wireless connection. Sure, there's apps like Remote and the Keynote controllers, but it seems like the iPhone-as-wireless-accessory idea is still relatively untapped on the App Store.