Is the Canon G2's, which should drop in price very soon, since the G3's have just come out. They're (the G2s) one grade up from the S40's. Price-wise they should be comperable with the Olympus 4040s; (I expect the G3's will be comperable with the 5050's).
I havn't had a chance to use either camera, but when I was researching cameras and considering buying one, I liked the spec-sheet for the G2 a bit more than I liked the spec-sheet for the olympus cameras.
On the other hand, the housings for the olympus are much cheaper... several hundred dollars cheaper.
The main advantages of the G2 over the S40/45 are:
a slightly brighter lens
better manual controls
a hot-shoe (gives you more options for using a flash)
The main drawbacks:
more expensive (by $200-$300)
significantly larger (still nowhere near the size of a film SLR, but the G2's are much bigger than the A-series or the S-series)
Canon makes a housing for the S-series, but not the G-series (non-cannon housings are available, but the Canon-made one is fairly inexpensive)
Jamie -- Staunchly resisting the tide of Oly-users