From the Pre-Order FAQ:
Why can't I pre-order using PayPal?
PayPal doesn't support charging orders on future dates. For this reason we're not able to accept PayPal for pre-orders. After a major beta version is released, there's an "open" period of about a week where you should be able to use PayPal (you will be charged immediately).
This is wrong. PayPal does allow you to get an authorization for a certain amount of money, capturing it at a later date. I know because I do it on my eBusiness. Barnes & Noble (www.bn.com) also do it if you pre-order games or other products from them using PayPal.
