I took my recalled Dell laptop battery to the Post Office this morning to see if they would ship it. It turns out that it is their policy to not ship batteries in general, and not just the exploding ones. The postal worker told me, “due to everything that’s going on, a lot of our policies have changed in the things that we ship.”
UPDATE (2006-08-24 19:43): I heard back from the USPS and some types of batteries are restricted. Here is a quote from the email:
Some types of batteries are restricted from being shipped through our system. For specific information, you will need to contact the Mailing Requirements Office in your area.