I've recently completed an iPhone app which allows you to use any PicoScope 2000 series USB Oscilliscope on an iPhone or iPod touch.
The app has been submitted to the app store for approval, and pending acceptance by the app store review team, will be available soon.
I'm not affiliated in any way with Pico, PicoTech or PicoScope - but they did help me out by providing hardware to test with.
The initial version of the app supports two oscilloscope channels and the full range of volt ranges and time divisions; three triggering modes with variable pre-trigger, trigger level and edge selection and AC/DC coupling on both channels.
This first release does not, yet, support the AWG features but this will be added in a future release.
You can preview the app on youtube here: