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:

- FYI - The first time I tried this I screwed up my entire 64bit distro bad enough to just give up and moved to 32-bit rather than re-install. Turned out to be a worthwhile decision overall, as I finally have the Flash 10 debug player working properly. I was also having other problems where Synaptic wasn't working. But that all took way too much time and I wanted to get my laptop in gear without wasting a day reinstalling vmware, android sdk, etc.

- The second time I figured it out. There's actually a pretty quick painless solution.

The first place I got tripped up was because I was getting an error message when I was trying to install ia32

I first removed sun-java6-jre in synaptic.
Then I installed ia32-sun-java6-bin.

That's it. Solved the error mentioned here.

Then to upgrade Flex Builder:

change your java version:
sudo update-alternatives --config java

then navigate to where your old flex builder was installed
aka cd ~/Adobe_Flex_builder_linux

ls in the directory and run the uninstall script there

try eclipse again to make sure its still good
aka /opt/eclipse32

download flex builder flex builder alpha 5 from adobe
chmod a+x flexbuilder_linux_install_a5_112409.bin

now run it as root
sudo ./flexbuilder_linux_install_a5_112409.bin

now hopefully flex builder installs. if you get java errors be careful and repeat the steps above closely. that's how I started to get screwed up last time. but then i got fb to install.

now you can try the Flex Builder script in your flex builder directory
sudo ./Adobe_Flex_builder.sh

but it failed for me with:

which is not helpful. So then I tried running eclipse normally

...And Flex Builder loaded perfectly when I accessed my Flex projects. Yay!