If I knew why, I’d fix it. I’ve updated the relevant plugin, and hopefully it’s a problem with feedburner, now…I’ll give it a few hours and see whether it solves itself.
Update: It appears to be a problem with the feedburner plugin and an apparent bug in the WordPress redirects. I think it’s fixed now. Rather than change settings that would break links, I’ve disabled the plugin entirely and used the instructions from here to get it working again, although it’ll need patching up once a proper fix is announced.