The 8051 architecture is sufficiently "yucky" that most compilers are difficult to write. I don't think there has ever been a "good" C compiler for 8051, for example (especially not a "free" one.)
What's the reason for wanting an 8051? If you really want lots of RAM, there ARE avr versions that support it. The Atmel XPlain eval board for xMega has 1MByte of RAM attached, for example (and I think someone had gotten some piece of the Arduino Core running on it.)
I'd rate the xMega as "most likely" to show up in new Arduino hardware; the ARM landscape is just too ... fractured.
There are other alternatives. We (ok "they") would need to be careful not to reach a price point where we'd all be happier running Arduino sketches in usermode under linux on something like a Chumby Hacker Board, or BeagleBoard, or etc.
(Hmm. User mode under Linux on assorted HW-hackable linux systems. THAT'S starting to sound like an interesting idea...)