it will not be a issue…. if you are well verse with 8051 you can easly switch over AVR
just refer to the link for avr tutorials
http://www.engineersgarage.com/embedded/avr-microcontroller-projects
you can use 8051 too just refer to the link for 80512 usb interfacing it might be useful for you
http://www.engineersgarage.com/forums/8051/8051-usb-interfacing