I don't use BaseCamp very often, so when I fired it up today I was informed that an update was available, and then that I could continue to use it while the download took place.
Problem was that there was no external indication that anything was happening, and I guess I shut BaseCamp down before the download finished.
I started it up again, went to Help->Check for Software Updates and did the download again. This time, I opened up the Task Manager, watched the network activity and waited 'til it finished.
Really guys, a simple indication of download activity wouldn't be too hard, would it?