My rootfs hovers between 10 and 20 megabytes free - sometimes I'll copy and then symlink stuff out to another ext3 partition when I need extra space.

The only time I've had more than 50 megabytes free was just after I flashed. I figure if you've got 50 megs free you're not really 'using' your N900 :-)

Or, you know, you're optifying it right. Mine is currently 50.0M free, according to "df -h /". I have the GCC compiler and a decent amount of relevant -dev libs on-board. A bunch of PHP-supporting packages, etc. Large ****, in other words.

The difference is that when I install a program that noticeably eats rootfs space, I track down the offending directories/files and move them into /opt/[whatever], and symlink from their original spots.

Honestly, the only stuff that needs to be on rootfs is the stuff that the N900 depends on to boot to the point of mounting the eMMC. Once that happens, symlinks to /home or /opt will work just fine.