(I should add for anyone reading this that Jake's also posted aboput wanting to do Skateboparding photography in the action section).
I'm sure you've already seen this, but you can compare the features between the 40D and 30D in our 40D verdict here:
http://www.cameralabs.com/reviews/Canon ... dict.shtml
Basically, both are great cameras for the kind of action you want to shoot, but I wouldn't bother with the 40D unless you want or need it's additional features: 2 extra mpixels, live view, so-so anti-dust, PC remote control, slightly quicker shooting, slightly bigger screen, improved weather-proofing etc.
Me personally? I really value Live View for example, so would go for the 40D, but others may not need the extra features and could save themselves 300 or 400 bucks which could buy an external flash or go towards a nice lens.