Well I had a go at the SW 20 and a couple of its non-sw siblings. They are all tiny and very light, about the size and weight of a mid size non-superzoom compact.
The shots taken underwater with the SW were impressive. Good colour, enough definition and contrast. But we were in crystal clear waters, shooting at around 5 to 10 feet in water clear down to over 90'.
On land, it is very much low end standard digital but better than most if not all still cameras using the movie mode. So I would still recommend it to the not so careful hiker if filming is not the main reason for the trip.
For you Ian, I would recommend that you have a look at the Panasonic HDC-SD9 (not the SD/Hard drive HS9)
Of course I did not have the Canon HD20 (my reference point) to compare it with, but the footage on a 50" plasma panel looked close to hd broadcast. This is a smaller (65x67x126mm) lighter (275g) and better (smoother,more detail) version of the SD5.It comes with 1x 32GB (!!!) SDHC card (4 hours at 1920x1080 p)