Automatic ranging level convertor?
The question is: How will your device know what voltage to use? The attached device will have to tell it some how. Will your device have a sense pin so it can test the attached device's supply voltage?

If so, the answer is easy. There are many voltage translators out there, e.g. the TI SN74LVC8T245. The input power supply will be your Atmel's 3.3v, while the output voltage supply can come straight from your device. This way, you will be able to connect a device with any voltage requirement from 1.65 to 5.5v, and it should just work.