duckyduck, let me guess: you have enabled the Fremantle extras-devel repository or perhaps also installed the last Fennec version from the Mozilla 3rd party repo. Meaning that you have gone beyond the safe path of the pre-installed Nokia Applications and Maemo Extras repositories.

If that is the case you are just suffering the consequences of it. Please find what are the applications you installed that dropped megabytes in your root partition instead of opt and MyDocs. File bugs against them at bugs.maemo.org and if they are in extras-testing give them thumbs down in the QA queue.

Anybody having an N900 today is a betatester, and this is how betatesters help fixing problems for the real users to come. Fixing a problem with a dirty hack in your local system doesn't help the Maemo 5 users and progbably doesn't even help yourself much (for instance, what happens when you need to make a software update?).

EDIT: also please share the list of apps you have installed before getting the memory full for easier detection of the most problematic and urgent cases. Thanks!