I'm very interested in the same thing.
I know that iPhone released support for the USB relatively recently (but I hate O2 with every fibre of my being), and I'd prefer to support nearer to Open Source anyway.
Friends are doing Android phone development, but they haven't dug into hardware interfaces yet. They did point out that I could build a new kernel image, and put whatever I wanted in, but it isn't clear that Android phones can support USB host or USB OTG. (I assume cr0sh is correct and current crop are USB slaves).
I might be willing to go none-Arduino to get USB OTG.
Also, I am waiting for iPad (friend should have one next week).
I assume Bluetooth is feasible as well as USB, and Bluetooth would let me develop and test on laptop too. Downside is Arduino BT seem to be silly prices.
Anyway, I'll keep watching this thread for info & pointers.
GB-)