I am seeing this same problem with all my video podcasts, including ones that are on disk, if they'd been downloaded with the newer builds.
For example, I'm subscribed to the DL.tv video podcast. I have it set to download the MPEG-4 version automatically. This podcast comes out 2 times per week, on Tuesdays and Thursdays. All of the files up through November 9th work fine, and are properly labeled in MC as MP4s and Video files. Starting with the podcast of November 14th, it is incorrectly labeled File Type = MP3 and Media Type = Unknown and they won't play (generating an unsupported media type error). Same goes for the Diggnation XviD (AVI) feed. I have it set to download automatically (one per week I believe). The one from November 4th is fine, but the one from November 12th is incorrectly marked as MP3 and Audio (instead of Unknown here) and won't play right.
Also, I have DL.tv's H264 (MOV) and DivX (DivX container) feeds added but set to manual download. If I try to play one of these that hasn't been downloaded, I get an error dialog (shouldn't this just download the file?). These also show MP3 and Unknown incorrectly. I tried manually changing one of these files (a DivX one) to AVI and Video and playing it. MC then hard-locked and had to be killed in task manager (that might have been due to the DivX container though -- I assumed they were AVIs until I just realized my mistake now -- still shouldn't hard-lock though). Manually changing the tags on one of the already downloaded files (the MPEG-4 feed) worked and allowed me to play it.
Deleting and re-adding the podcast feed seems to have fixed the incorrectly recognized media type for the H264 feed, but has not for the DivX feed (which still lists MP3 and Unknown for all of them).