given that some usb adapters actually use avr pics, is it conceivable that a duino style bootloader could be written that can do all the "gruntwork" of a usb connection? So that it could receive a new sketch via usb with a simplified interface circuit (like a couple transistors)?
I know practically nothing about bit-banging USB, but I know that the USB protocol is pretty complicated and it's difficult enough fitting a simple serial bootloader in the limited (2k) bootloader space. I'd be very surprised if you could fit it all in 2k.
- Ben
I did find this after seeing some other threads here
http://www.obdev.at/products/avrusb/index.html
So it is basically a few diodes/resistors and less than 2k of code to get an atmega168 to communicate over USB without extra chips 
But, this would only be good for loading new scripts, once the script is running, I would expect the usb stuff needs to be shut down (takes significant resources to be usb), so it couldn't be used for script serial communication. And it would probably be a manual reset situation. Simplifying the hardware is worth something, but I don't know if it is worth complicating the load procedure or losing usb communication at script time.
Actually, USBTiny and AVR-USB use about 1400 bytes for the base functionality, so with some careful coding it just might be possible. Of course USBTiny currently wants a 12MHz clock only, which AVR-USB can run at the Arduino 16MHz frequency among others.