This appears to be a pymaemo-optify bug, where the mounting setup is not done properly on boot. If I install all these python packages in the right order, then things work fine as long as I don’t shut the device down. If I do shut my N900 down and reboot, python produces the error I pasted above. I guess I’ll have to go searching through the pymaemo-optify .deb to find the startup script and ensure myself that it gets executed on boot.