OK. So for anyone else having this issue, when Adobe decides to piss on us Linux Flash developers again:
This time it was making my Flex Builder Alpha expire. First it took hours ( and two unhelpful Adobe reps ) to realize that there was a new alpha available for Flex. I thought they were trying to make me buy a license. So I figured, oh, cool, it'll be a cinch to upgrade. But I was incorrect.

Upgrading Flex builder linux to alpha 5, on 64-bit Ubuntu 9.10: