The iPhone Blog reports that the iPhone 3.0 beta firmware may close a loophole known as "account sharing." The practice allows one App Store account holder to share their login information with another user so that the second user can freely re-download apps purchased by the original user to a second iPhone.
With the new 3.0 beta firmware installed, users get a new warning dialog (pictured, right) when they attempt to download an application over-the-air after the first time.
Apple does however allow you to re-download applications an unlimited number of times to a desktop computer - a definite inconvenience for account sharers. As can be seen in the warning dialog, your options are to re-download the app for free on your computer, or to buy it again.
Tip: The iPhone Blog