Hi Swordfish,
All good points made so far. If the Ikelite housing is your only option (financially) then the Nikon, Canon, and Oly cameras will all feel the same underwater. They'll all be in the same clear box afterall, and realistically, the Nikon, Oly, and Canon entry level viewfinders are all the same (poor for underwater use).
For the same price, as the E500 kit, you can get a Nikon d70 or Canon Rebel XT. These cameras have greater support for underwater and topsides use, including lenses, flashes, accessories, and housings and ports.
Tio on dd.net pointed out that there is a good lineup of underwater lenses for the E-500 but at what cost? Since Nikon and Canon are supported by 3rd party lens manufacturers, you can get a $500 wideangle zoom like the Sigma 10-20 as opposed to the $1,250 wide zoom made for the 4/3 cameras. That's a big cost difference.
And if you're willing to spend a few hundred dollars more on a housing, you've suddenly gone from two choices for the E system (oly maybe, and Ikelite) to more than a handful of housings to choose from.
Cheers
James