For some unknown reason, I lost a few episodes of a podcast, so iTunes showed them with an exclamation mark and no obvious way to re-download them. Searching the web for a way to make iTunes to re-download them came up with lots of comments about having to unsubscribe and resubscribe. I didn't want to do that, because I still had a bunch of episodes that I wanted to keep, and I didn't want to have to download those again.
After playing with iTunes a bit, I found a pretty simple way to do it. First I clicked the triangle to expand the podcast series and then deleted the missing episodes (the ones with the exclmation) by selecting them and hitting the backspace key. After that, I collapsed the podcast series by clicking the triangle, and then expanded it again while holding the option key (I think it would be shift on Windows), causing iTunes to update the podcast and add back in the episodes I just deleted.
The re-added episodes had the 'Get' buttons again, allowing them to be re-downloaded.
Mac OS X Hints