I saw this link on David Tribe's blog.
Free radicals (e.g. reactive oxygen species) are highly reactive chemicals that play various biological roles, including acting as signaling molecules. They're most famous though for reacting with (and breaking) cellular machinery (due to their highly-reactive unpaired electron) and contributing to cellular stress and disease. Antioxidants deactivate radicals, and therefore protect native proteins, lipids, DNA, etc.
The article cited above describes a new study that found that reactive oxygen species appear to play a protective role against Type II Diabetes. The implication is that consuming too many antioxidants (e.g. loading up on vitamins) may backfire.
This just goes to show how much we don't know about how our foods affect our health (and why I don't believe in vitamins). I'm just going to keep eating a reasonably diverse diet and not worry about the consequences.