Don't want to wait for multiplayer in Portal 2? Some modders are privately testing out Team Fortress 2 equipped with portal guns, and released this video to show how far they've come with the concept.
While anyone on the board can pass through a portal, whether they laid it down or not, the portals can be destroyed. (Coding to allow shots to pass through them would have been prohibitively complex.) They also disappear when their owner is killed, or if he reloads. And it looks like the progressive velocity of falling through two portals keeps anyone from being stuck in an infinite loop.
Destructible portals also allow a team to prevent its opponent from constantly beaming in behind them. They do provide access to otherwise unreachable parts of the map, and deliver a heck of a speed gain. As an offensive tool, catching your foe in a loop - see the 1:47 mark - lets you toy with and ultimately sucker-punch him.
Team Portress [YouTube]