Wow, fantastic budget! At the risk of stirring up a hornet's nest, some people on the net have questioned the long term viability of the Olympus 4/3rds system. The smaller sensor is more likely to introduce picture noise because it has less light to work with. As I remember it, part of the rational for choosing a 4/3rds format was that that was what televisions and computer monitors looked like. Well, the world is very rapidly going widescreen!
Current rumour suggests the Canon 40D will be released in late September or early October. That may be a load of tosh but you are in the happy position of being able to wait until then before you decide.
You have already spotted how important it is to buy into the correct lens system and with your budget you will have enough cash left over to buy a more than decent body whichever system you choose. On the assumption that you are comfortable with one or more camera bodies from each company I suggest you concentrate your research on lenses initially and then choose the body.
There is lots of good advice on this and other sites about what various lenses are best at. A fast lens can essentially do two things that a slower lens with image stabilisation can't do. It can give you a pleasing out of focus background and it can help freeze motion because of the faster shutter speed it allows.
At your budget, and at the risk of upsetting the Nikon fanboys, I would choose the Canon 5D. The full frame sensor is a big part of this - read Gordon's review
for more reasons. My lens choice would be the EF 24-105mm f/4L IS USM. This is actually the kit lens (
) in the UK and the two together cost around $4000. I own this lens for use on my XTi. On a full frame sensor it covers you from wideangle to just into telephoto. Although the 5D body plus an EF 28-300 f/3.5-5.6L IS USM is just in budget and is enormously flexible I think I would find the size and weight a bit much unless I was on safari.
So with a 5D and an EF 24-105mm f/4L IS USM there is more than enough left in the budget for a flash unit of your choice and other toys or, straying outside your brief of just one lens, an EF 70-200mm f/4L IS USM.
You might have noticed that I already have the EF 24-105mm f/4L IS USM and EF 70-200mm f/4L IS USM lenses so would you like to guess what my dream body is (apart from Kylie