Micro-Four-Thirds system cameras have interchangeable lenses and relatively large sensors in fairly compact mirror-less bodies. That is, near DSLR image quality and HD video in a relatively small casing.
So far however, only Panasonic & Olympus have presented µ4:3 cameras & lenses. Meaning the selection's still rather limited, though slowly expanding and quite promising.
Most importantly, several third party manufacturers have already started developing housings, e.g. 10Bar, Athena, Nautilus & Seatool.
UPDATE: Inon's X-2 housing can now also waterproof the GF1 to -75m.
