>> I realise anything is possible, but the true cost of rolling your
>> own bit-bang code may not be worth it, in terms of time spent
>> and project delays etc
>
>No, it is not possible, no matter how much time and money you have.
>The full-speed USB bit rate is equal to the fastest PIC's instruction
>clock.  I doubt it would even be possible to bit-bang low speed USB.

I think there is an app note for bit banging USB on an AVR processor. Let me 
look at my library.

Yes, here we go - AVR309 Software Universal Serial Bus, doc 2556. Supports 
low speed (1.5Mbit/s), implementation runs on small AVR devices (2kB and up) 
and has full Windows USB driver, DLL library functions and demonstration app 
in Delphi available for download.Scanning through it, there is some good 
looking theory on USB, and example circuits using AT90S2313/ATtiny2313 and 
ATmega8 The copy I have is preliminary, dated 07/05.

http://www.atmel.com/dyn/resources/prod_documents/doc2556.pdf is a later 
issue I just downloaded, dated 02/06, and no longer preliminary.

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