I didn't realize this was a recent event; otherwise, I would have reported this, too.
Since the release of the latest version (438), I've done two semi-large imports. I don't do the auto-import thing (I like to maintain full control), but the first night I imported one large directory tree (120,000+ images and video clips), and the second night I did another directory tree (~10,000 images).
For the first run, it took 6+ hours to import, but even after MC reported that it was done, I still could not close MC. ("MC is still importing. Are you sure you want to abort this process?" [paraphrasing]) I canceled exiting and let it continue, all the while a JRWorker.exe process kept popping up in my task list. It was annoying only because it would
always take the focus away from whatever app I was running, be it Firefox or even a game.
I just left MC open all night long, and it seemed to finish (building thumbnails presumably) by morning.
For the second run, same thing. The import only took an hour or so, but again MC had to churn ALL NIGHT doing whatever it was it was doing. (Again, building thumbnails I presume.)
Again, I didn't report this because I thought this was "normal," but I do think that you need to provide some sort of status/feedback to the user. A time estimate would probably not be doable, but at least something like "Building thumbnail 23,434/120,000" or something.