Hmm, that is odd. They were working just fine before the latest update, and I definitely had PDN set as the default program for opening TGA files...
Do you have a link to an older build that I can download and install for testing purposes?
Update: As a workaround, I installed SageThumbs, which restored TGA thumbnail functionality to Explorer. I then had to reset PDN as the default program for opening TGA images, and thumbnail functionality remained intact. As a friendly suggestion, perhaps you may want to add a warning in the installer next to the option that allows PDN to be the default program for TGA files that associating TGA filetypes with PDN may potentially break thumbnail functionality in Explorer. Or perhaps you can add TGA thumbnail support in the next patch.
I don't mean to be pedantic on the issue, but the point is you modified thumbnail related code prior to this latest version, this post was created as a result of others seeing a similar issue I was having with thumbnails, and now TGA thumbnails require a workaround in order to restore previous functionality until you officially support it in PDN itself. So I can't simply accept that "TGAs have never had thumbnail support" even if you are only speaking about PDN.