Since this is one of the most intriguing elements of each World Cup, I figured it deserved a thread of its own.
For me the most likely source is definately Bangladesh. They have a decent number of hit-or-miss yet dangerous ODI players like Mortaza, Ashraful, Aftab Ahmed and such, and in a group with Sri Lanka and India there's an outside chance that they will quality for the Super Eight round.
Kenya are generally a pretty decent ODI team, and that may continue into this World Cup, though I don't see that they've developed much in the intervening period since 2003. Ireland seem like a decent team as well, even if they are missing Ed Joyce now.