Mystery of Tyrannosaurus Tiny Finger.

My hypothesis is that they used them for display and for help carrying carcasses.

Proably for holding on during mating, and likely to help get itself up after laying down.

I think the arm were mostly used to help in copulation, but also for close encounters with some prey animals.

The males should have used their arms to help stabilize their mates during copulation, to keep in contact with her, but also maybe to keep her from falling flat on her face.  The females might have used them to make sure she didn't fall as well, as she was below her mate the entire time while mating.

The second use stems from this theory:  for large prey, T.rex probably did not simply grab hold of its prey and bite down.  I don't care how strong a T.rex's skull was, it's body weight could still deliver more force when coupled with its bite.  T.rex might have actually tackled its large prey to the ground with the help of its jaws to make sure the prey was in an indefensible position.  Once the T.rex was on top of it's prey, those arms would have stabilized at least part of the prey animal so T.rex could kill it quicker.  

In both cases, the T.rex had to get extremely close to another creature, which could just as easily spell disaster for it if the situation goes against its favour.

Or they were used for all the reasons said. Mating, getting up after sleeping, battle, and anything else. If T-rex had those arms(which were strong for their size), it probably used them whenever it could.

Oh, and I think this is appropriate for this post...

That's a good image, yes, very appropriate for the situation.

This image is very good.  And it acually clarified something I was debating with myself for a while:  were the functional fingers of late Cretaceous tyrannosaurs the thumb and index finger, or the index and middle fingers?

Definitely the thumb and index finger.

Rex Fan, I think your answer was the best out of all answers.  If T.rex could use its arms for anything, it would have.

It surely was multi-functional,  so, I agree with what was stated by RexFan earlier. (It just seems those days you're making some iterensting paleontologic debate topics, while I'm sleeping. So I don't have the time to say my opinion, as all is already said. :D )

Its a very favorable theroy that Rex was at the front of the "Eveloution Arms race" And by losing long arms (which could be like Spinos as an example) Would provide something to evolve in its place and give it a sort of sharpened blade for prey...

But of course they also probably helped for balance, and by losing them for better and possibly more combat worthy weapon, Much like a humans big toe, thats right, the only reason you have it is for balance, werid I know.

I think rex fans got the right idea, just thought i'd throw this up there too

O.k I think the arms use to have muscle inside them but in the evolutionary chain the muscle transfer to T-Rex's jaws giving it more biting power  making it having the strongest bite of any dinosaur!