Hi Peter, welcome to Wetpixel.
If you're going to buy an SLR type underwater camera system, you're making a considerable investment. I urge you to consider that you will be using your kit as a SYSTEM, and not just look at the price of the camera body and two lenses.
How many housings are or will be available for the Olympus E-500? Of course, there are none now, but even after a year, I bet you will have 2 or 3 choices max. If you buy a Canon or Nikon camera you will get something like 8 or 10 housing choices! More choices means more flexibility for you and most importantly, COMPETITION leads to lower housing prices.
I don't think your point about the lens crop being confusing is a valid one. For Nikon, there is a 1.5x crop for all of their cameras. For Canon, any reasonable priced camera has a crop of 1.6. The Oly has a 2.0 crop.
The difference in lenses comes up when you look at the three lines. Currently, Canon and Nikon offer literally hundreds of lenses to choose from, and these are mirrored by Sigma, Tamron, and Tokina. What this means is more COMPETITION leading to lower lens prices. Oly has no competition right now, so they can charge whatever they want. They charge a LOT and offer lens rebates to make their Zuiko lenses look more reasonable, but look at comparably prices lenses by Canon and Nikon and they are much less expensive.
Another comment about the lenses is that Nikon and Canon offer special lenses for topside use that incorporate image stabilization, perspective correction, and other nice features. Canon even makes a macro lens that does 5:1 magnification! These types of lenses may seem like "specialty" items, but just wait until you're shooting a telephoto lens in low light and you'll want that image stabilization. Olympus may try to build this feature into their cameras eventually to get around the patent, but the IS is much more effective when specifically designed and built into the lens.
I'd encourage you to broaden your search. You may find that when you look at a D50+kit lenses or a 350D+kit lenses, and then add in the HOUSING cost, you might come out cheaper than the Olympus. You said that you:
like the thought of starting with an E500, buying glass as my budget and skill improve and being able to use the same lenses on a better Oly body.
There's no reason you can't do this with a Canon or Nikon camera. Start with a base body and kit lens, buy a few better lenses, then later upgrade bodies to something better. What's nice is that if you buy a base model Canon, you have at least FOUR other cameras to choose from in the future.
Cheers
James