Was doing this on a vanilla n900 with a freshly flashed emmc and firmware, so all messages were imported. It was crashing when I scrolled a few dozen messages back in the conversation view and encountered a message that probably had something in it that the app couldn't handle.

Probably some special chars. Or then I butchered my .csv file. I also have this behaviour with the other versions of the smsimporter, btw. I could have tracked it down to the exact messages that caused the app to crash, but in the end couldn't be bothered and thought I'd just wait for a decent importing app, since there are apparently some people already working on it.