Good question. Way back wen I invested quite some money into the bounty system and suggested that bounties should be contingent on any progress over time. In addition, the bounty should only be paid by the sponsors at the time the bounty is actually claimed.
So far, nothing much happened. So, I focus my attention on Qt, Java 6 and other projects with noticable progress and transparency over where my money goes...
I've considered taking on a few of these bounties, but there's no way to tell which ones are still valid. Ideally, the ones listed as open should be good, but it seems as though they need to be updated.