I think the Amiga is still too complex to cheaply emulate in a single chip. But, with the Sega Mega Drive PlayTV out it looks like they're getting closer (I haven't seen if there's a real 68000 and/or Z80 in there or if they've shrunk it down to a single custom chip).
But, since the C64DTV is released on 11/26 that will keep me happy for some time. There's already talk of hacking on mass storage devices, pc keyboards, etc. to make it a super C64.
Edit: I found pics of the inside of the Mega Drive PlayTV and it is in fact a single chip. The only two chips on the board are the 32Mb ROM for the games and the MD guts (assumably the 68000 + Z80 + whatever other custom chips were needed). Unfortunately, it only has 6 games.