After a dist-upgrade from Gutsy to Hardy on our webserver, TYPO3 was suddenly broken. The reason is a bug in PHP 5.2.4. There is a workaround for TYPO3, but unfortunately the patch is still pending in the core list.
Oliver Harder already requested to commit a patch and it works.
So if you run into trouble with Ubuntu 8.04, download the patch to your typo3_src directory and patch your sources:
patch -p0 < 0006158_v2.patch