Seriously, they're probably vestigial.

One thing that is quite bizarre about the tyrannosaurs are their tiny little forelegs. These actually became smaller and smaller as the creatures evolved. The two last and most advanced tyrannosaurs - the medium-sized Albertosaurus megagracilis (assuming it is a valid taxon; most modern accounts consider it a sub-adult T. rex) and the famous Tyrannosaurus rex - had the smallest forelimbs (proportionally speaking) of any dinosaur, and it is likely that if their evolution had not been cut short by the terminal Mesozoic catastrophe they would have lost their arms altogether. Like ground-living large running birds, the big theropods really had no use for their forelimbs. They acted more like huge flightless birds of prey, than the overgrown bipedal lizards of popular imagination. Interestingly, after the extinction of the dinosaurs, a number of real flightless birds - Diatryma, Phorusrhacos, etc - took over the ecological niche of the theropods, becoming huge-headed, fast-running predators, 1.5 to 2 metres or more in height.

The progression of vanishing forelimbs is apparent in flightless birds, too. I suspect that birds like ostriches have maintained their wings solely for display purposes. On lower-profile birds like kiwis, they're almost gone.

On the other hand (heh) some researchers have found that the Tyrannosaurus forelimbs were quite strong for their size, which implies that they were used for something.

Heck, maybe Tyrannosaurs had feathers here and there, and the limbs had pretty colored display fans.