i have used and implemented this as various interfaces CDC, HID, etc. and have found to be most rewarding and instructive .
however to answer vinod's original question it is not possible to implement this in the pic even if the pic runs on the same frequency as its atmel counterpart ( AtMega8 here) as the atmega executes one instruction per cycle while the pic requires upto 4 cycles and it is just about enough thoroughput to implement a low speed USB interface.
Thanks for reminding v-usb respectively avrusb, which has been the previous project name. It's the software, I meant in post #6, but I didn't remember, that I already had downloaded a 2008 release of the project. Your basically right about the different MIPS to clock ratio, but I think, it's not completely impossible to implement a similar thing with a sufficient fastly clocked PIC. As I already clarified, I don't apply for the job.
I think we need atlest 12MIPS controller for this project lots of PIC working at 5MIPS and other higher 12 or more MIPS controller already have USB controller on chip like PIC 18F2550.
for implementing software usb on chip you need 12 things to be done in 12 clock cycle.