I bought the Olympus a month ago and I love it. It has so many amazing features, one of which is the Supersonic Wave Filter to clean the sensor. I've had no dust yet after a month of almost constant use.
If you're looking to go into aviation photography, as I was, get the Olympus just for the reason that for 800 USD you get the camera, a 14-45mm lens, and a 40-150mm lens. As Jorge1812 said, it has a 2x crop factor, but since the low end starts at 14mm your base will be 28mm, only one more than the 350D. And then, your maximum is 300mm. For the 350D you need to buy the kit with the 18-55 and then get (if you don't want to spend a fortune) the Sigma 70-300USM which is somewhere around 200 dollars. You'd end up paying more than the Olympus kit, and you'd have the big gap between 55 and 70mm.
As for adding lenses later on, keep this in mind. Olympus lenses are on par with Canon and Nikon quality, but they are VERY, VERY expensive. An Olympus 50-200 is 850 dollars, where as the same lens made by Sigma for Canon or Nikon camera is around 550.
Just my two cents, I'll leave the decision to you, but you should know that I am very happy with the Olympus.
www.stellaryear.com: Canon EOS 50D, Canon EOS 5DMkII, Sigma 50mm 1.4, Canon 24-70 2.8L II, Canon 100mm 2.8L, Canon 100-4