I guess the 8 fps uses the mechanical shutter - which is amazing if the mirror is flipping up and down with it as well. The Pentax can only manage 3 fps like that - not bad, really.
Burst modes use an electronic shutter, so it's more like a video camera in that mode, with the performance being purely down to the electronics rather than needing super good mechanics (as well as good electronics) like you must have in the 1D. The resolution is bound to increase in future without an increase in bulk as hardware gets increasingly more efficient.
Just had a look at the 1D prices via Google Products