We haven't yet got a confirmed date for the Open Beta, but that doesn't mean we can't speculate on when you can expect it. So you could absolutely preorder, get a beta code, and then cancel your order after the beta finalises if you don't like the game at no cost. Remember, you're getting early access to the Open Beta with your preorder, and you don't have to pay anything until the game ships in late October.
For now, here are some quick links if you just want to lock your preorder in ASAP.
Preorders are now live for Call of Duty Modern Warfare 2, and there are a couple of different versions to choose from, so make sure you check out our full MW2 preorder guide before deciding. Preorder Modern Warfare 2 for Early Access to the Open Beta