As I somewhat had expected, there is a decent way to tag photos in Windows. What I did not expect is that it is built in to Windows itself (Vista and 7, anyway). You can edit tags from the properties dialog on images, doc files, and a few others (but not all files, for unknown reasons). It's easy and the search bar in the upper-right corner actually does a pretty good job and will recurse into subfolders. It'll even pick up the camera model; I did a search for "rebel xsi" and it tossed me back everything taken with my new camera.
By default, it'll search on filenames as well, but you for prefix it with tag: and just search the tags. For example, I tagged a few things and then did tag:christmas and got what I was looking for. The only obvious feature I see missing is that you can't right-click on a folder and add tags -- you have to select a group of images first. And, of course, there is no tag merging or splitting, but that is minor and would probably never be used anyway.
I think I will play with this for a while and see if it is a usable solution to my never-ending image tagging project. If it does work decently, there may still be a need to write a program that'll do a nicer-looking search, import/export.
Edit: It's kind of obvious, but I forgot to mention it -- that there is no sort of tagcloud or anything. So, while the tagging might work great for searching, you aren't going to be able to visually see what you take pictures of the most or do any kind of "data mining" on tag frequency or tag relevance. Anyway, I'm gonna play with the hard part -- tagging 100,000 pictures!