What constitutes a touchdown

Just the tip of the ball has to cross the front of the line.

Cycloptichorn

and if a player is tackled while running it into the endzone, the tip of the ball must cross the imaginary plane before the ball-carrier's knees touch the ground...

Just the ball needs to cut the plain.

But what if the receiver's feet are in the endzone but he, and the ball, ar leaning forward out of the endzone? The ball didn't break the plane but the receiver is technically "down" in the endzone. The rulebook doesn't appear to address. FYI, this actually happened in the Ravens/Steelers game tonight. Fortunately, I'm a Steelers fan but I am looking for some clarity on the rule.

The rule book states that even if the receiver's feet are down in the endzone, if the ball does not cross the plane, it is not a TD. After extensive review, you could probably just about argue it did cross the plane in the Ravens-Steelers game, but it was too close to call so the original decision on the field should have stood. Watch this vid http://www.nfl.com/videos?videoId=09000d5d80d601d1 at about 0:15 you will see the ball appearing to cross the plane. Hope that clears that one up, it's strange how video replay decisions can lead to even more controversy than not using them sometimes!