The best place to start in uderstanding the 8051 processor is with the three 80C51_FAM_*.pdf documents at:
http://www.semiconductors.philips.com/products/all_other.html

Here is the description of the RI flag from the Programmer’s Guide and Instruction Set document:
"RI (SCON.0) Receive interrupt flag:
"Set by hardware at the end of the 8th bit time in mode 0, or halfway through the
stop bit time in the other modes (except see SM2). Must be cleared by software."
(my emphasis)

You can read-up on the full details of the 8051 interrupt system in those documents, then get your T-Shirt:
http://www.thinkgeek.com/stuff/apparel/3239.shtml