I narrowed this down to SD cards. I had bought a pair of cheap Lexar 2GB cards and both of them would cause maemo-mapper to crash at pretty much the same spot when downloading maps. I tried a 256MB Kingston Elite Pro that I had lying around and didn't have any problem filling up the card with maps. I just picked up a Patriot 2GB "Extreme Performance Plus" card and it is performing fine as well.

So, it was a hardware problem. I don't know, however, whether the cards are defective (although I was able to fill them up by copying video files from my PC) or they weren't fast enough.

-- Hal