Does the 5D camera really use a Digic II chip like all the other Canon cameras?
i highly doubt it's the exact same silicon, but it's the same basic processor, yes. developing ASIC chips like DIGIC is very expensive, so companies will tend to design it for the widest possible range of applications. in this case, Canon is using the same processor design in everything from the $150 PowerShot A series to the $8000 EOS 1Ds. i would guess the D-SLR cameras have higher clock speeds, more on-chip memory and registers, wider busses etc. to deal with the increased throughput though... similar to how nVidia will make low-end and high-end versions of the same basic GPU core.
How is the CMOS sensor any different than the 6.2 MP sensors already used?
if you're referring to 6MP sensors in compact cameras, then it is a fundamental device difference. the compact digicams use CCD (charge coupled device) technology, while CMOS (complementary metal oxide semiconductor) is a newer technology. in current implementations, CMOS has superior noise and dynamic performance, but are more difficult to implement for moving video. they are also more expensive to produce.
also, the sensor is physically much, much larger in the D-SLR cameras - in the case of the 5D, it is the full 36mm x 24mm size of a 35mm film frame. non-full frame D-SLRs are usually APS-C size, which is about 23mm x 15mm. the 1/1.8" sensors in most small digicams are a paltry 7.2mm x 5.3mm. smaller sensors for a given resolution translate to smaller pixel sites, which means less light and less sensor area per pixel, which means higher gain and lower dynamic range. this in turn means more noise and lower overall image fidelity with a smaller sensor, and is the primary reason why the image quality of a digicam will never be as good as a D-SLR. and of course it's a question of optics, though i think the lenses in many of the compact Canons are actually pretty good, all things considered. fixed lenses can be optimized for a given sensor and retrofocus geometry, something that cannot be done as easily with an SLR setup.
Just thinking if this is a glimpse of things to come in compact cameras (except the the heavyweight optics).
in terms of sensor size, no. in terms of sensor technology (CMOS), maybe. in terms of overall image quality... the current compact digicams are fairly comparable to the DSLRs from a few years ago, but i think they are starting to hit a point of diminishing returns, at least with the current sensor technology. but obviously technology will continue to improve across the board.