The only cases where I personally believe bounties have really made a difference are where it was a very highly in demand task that generated a lot of money for something that wasn't that earth shatteringly difficult. Like getting N64 emulators ported to XBox, which generated something like $2000 several years ago.
In order for something like that to happen to begin with you've got to have a community with a very high users to skilled developers ratio, or at least one where most of the skilled developers are already selling what they do like on iPhone, Symbian, or Windows Mobile. Nothing like this community, where there aren't that many users and there are a ton of skilled developers.
Bounties in the past, say for GP2X, haven't generated any decent cash because people don't really have confidence that it'll go anywhere (and rightfully so). But this doesn't mean this community isn't willing to put up cash - quite the opposite, for evidence see how much zodttd has received (and I have gotten a portion of) from "donate to beta test" money, even in this community (as opposed to iPhone's where the volume is just huge). Looking at this from the opposite end, that is, using developed programs to entice money instead of using money to entice program development, things look very different. Let's say, for example, Ari64 and Adventus got together and did the Mupen64plus ARM recompiler and 3D acceleration together, but then decided to hold it hostage for $10,000 before releasing it, while getting someone very trustworthy (say, ED) to demo it and vouch for it. They'd get a lot of bile for this, but they'd probably also raise the money, with dozens of people showing up to pay tens or even hundreds of dollars.
Actually that sounds like a fun idea, I should have tried it on PSP ;p If you can make 100,000 newbs pay $1 each you'd be sitting pretty well. I doubt you'd be able to though.