There are a number of Apps that let you print to networked printers using WiFi. Though they need to log to the e-mail themself rather than the data on the phone itself. (SDK restriction no doubt)
Print and Share does this.(and can print web pages from its own browser)
Air Sharing let you print PDFs nad photos you and saved into the Apps folders via wifi, or your MobileMe or other on line storage service.
Print Magic will print photos from your naitve iphone photo App.
Presumablly with the opening up of the SDK that Apple promised this should also be able to be done via bluetooth sometime soon.
I would hope by iPhone OS 4.0 this would be a native function on the phone.