My random-guess would be Shaun Pollock - and kinda hope so, too, because if he is there's a good chance SA have won the thing.
That's actually a pretty good guess. Pollock's in great form with both bat and ball lately, and South Africa are doing well enough to at least get deep into the tournament. Unless they have an absolutely insane tournament I'd generally rule out specialist bowlers from winning player of the tournament. It usually goes to batsmen, particularly those that score quickly and dominate matches, or all-rounders. But a bowler who is consistently good and plays a good innings or two would be in with a big chance, and Pollock could do that.
Aside from Pollock, I'd say both Ponting and Hussey are reasonable chances. Two of the best ODI batsmen in the world, in a team that is likely to play a lot of matches, and Ponting at least is in incredible form. Hussey may bat too low in the order to win it, but if he only gets out a couple of times that probably won't matter. There's also the chance he'll come up the order a bit, particularly in chases.
Pietersen is another one who definitely deserves mention, though England would probably have to make the semis for him to win it. If they do, he'll likely play a big role in it.
Gayle for my 5th bet. Contributes with both bat and ball, scores quickly and has a big presence on the field, and if the West Indies go a long way he'll probably make plenty of runs.
edit: Actually, instead of Hussey, I'd probably have Symonds as my 5th player. If he gets fit in time I can see him having a big impact.