Hi
I'm author of opencores 8051 IP core. So if someone is interested in it, i can give him any help he'll waned. This core was tested in xilinx fpga and have all peripherals. It's written in verilog.
If you don't have access to home page i'll can send it through mail.
project home page is:
www.opencores.org/projects/8051

I know for two more free cores on web:
http://oregano.at/services/8051.htm and
http://www.cs.ucr.edu/~dalton/i8051/
they're both in vhdl.

So if you have any questions, suggestion ... send it to me:
simont@opencores.org

bye
simon