But, this doesn't prevent this at all. Even if you have to "re-purchase" apps, how exactly does this prevent people from "sharing" apps? My wife and I do the same and this new policy means nothing to us on terms of "sharing" apps. This doesn't prevent anything, but just produces inconvenience for the end-user.
You are allowed to share apps based on the 5 authorized computers. However, with iPhone 3.0 you can enter an iTunes username and password into the iPhone itself. Because of this, you can have an application which was paid for only once running on dozens on iPhones - clearly something which could cause developers to abandon the iPhone development. Apple's new policy ensures the five computer limit.
As it currently seems to be working, if you buy the app on iPhone "A" you can re-download the app to iPhone "A" unlimited times without paying. If you try to download the app straight to iPhone "B" however, you will be asked to re-purchase it or sync it via computer - this procedure ensures that the limits which have always been in place are adhered to.
For users who sync their apps to only one iPhone, this should not be a problem. However, once you upgrade to a new iPhone you will need to use your computer to sync - or perhaps Apple will have a way to make your new iPhone the original device.