For a really quick start with USB, you could try the USBMOD02 module from FTDI:
http://www.ftdichip.com./FTEval.htm
A big plus is that they give you free (and royalty-free) PC drivers with it.
It was featured in Electronics & Wireless World a few months back, and has been mentioned here; eg: http://www.8052.com/forum/read.phtml?id=22061
Search for "FTDI"

Triscend has a USB+8051 Reference design, and I think Texas and Atmel have 8051s with USB.

There's quite a lot of 8051-related USB info on the free Keil CD - possibly also on their website.
The index is /Communication/usb/usb.htm on my CD.
(note that I used forward slashes for fear of the forum swallowing the proper backslashes)

Don't forget www.usb.org (go to the "Developers" section) and, of course, a search for "USB" here!