The ColdFire chip will run SOME 680x0 code natively, but some code would cause interesting things to happen
the ColdFusion accelerator has trapping routines for the unsupported code and gets the native replacement to run instead. (I think... I'm not on the development team, just want one when they're finished!)
The accelerator board is not a seperate computer, just has as much of the new stuff local to the CPU bus, as possible - so you can use 100MHz SDRAM, EIDE drives, Firewire, USB1.1 and AC97 sound chip, all at full speed, as well as any accesories plugged into other ports on the amiga. Kinda like the CyberStorm Mk II cards, that have RAM and an accessory connector, which could take fast SCSI II adapter + others (most of which never made it to market
The prototype version shown on the main page is suitable for A3000, and A4000, desktop or tower, and plugs into the CPU slot on the Amiga motherboard.