Steven Knapp has links to this & some other VHDL resources on his 8052 home page.

"The 8051 core is still a work in progress."

There are several commercial 8051 cores available (which doubless accounts for some of the 8052's popularity), but all the freebies I've seen seem to have some significant parts missing; eg, no interrupts! :-0

I think someone mentioned on the Keil forum recently that they had a free 8051 core?