If you havn't come accross it yet the place to find out info on USB for the 6502 is http://microusb.org
While developed for the Atari, the circuit and code should be very usable on the Apple II.
A few summers ago another Apple II USB project was started called the anycard by Jeri Ellsworth and Hans Franke. I havn't heard much about in last 6 months - perhaps there will be another update @ KFest this year.
Anybody else have an update on that project?