I just noticed Adam Bittner already wrote about this, but his take is different enough from mine that I think it's worth going with twice. Apologies for the repetition.
Dave Cameron of FanGraphs argues that the Pirates should consider buying at the MLB Trade Deadline, because the potential rewards of doing so are great. Although the Bucs aren't quite as good as their record so far this year and aren't a great bet to make the playoffs, he argues, the rewards of a playoff spot in terms of attendance are so huge that the risk is worth taking.
Assuming the Pirates don't go on a 10-game losing streak in the next couple weeks, I think this argument as a lot of merit, but it's all a matter of degree. It wouldn't make sense for the Pirates to go trading someone like Jameson Taillon, as Cameron himself suggests, and they really shouldn't be trading top prospects of any stripe. They have a future to think about that, and while a 2011 playoff berth also could help pave the way to a brighter future (by giving the Pirates more revenue to work with), they shouldn't be sacrificing a prospect like Starling Marte or Stetson Allie, say, to pursue that goal, however noble.
As I've said here and elsewhere, though, the Pirates have a ton of lower-watt prospects who they really won't miss much, especially since they stand to lose a couple in the Rule 5 draft this winter anyway. Assuming things go well, they can consider trading those kinds of prospects for offensive help. They might also consider creative trades where they ship out prospects for young players who can help immediately and in the future. This trading deadline could be a very interesting one for the Pirates.