Applications cannot be redirected to the external SD card without a fair amount of work, and if you do it you won't be able to safely remove your SD card without shutting down.

Not saying it is impossible, but difficult and problematic.

The N900 acts like a regular Linux distribution and uses a package manager to install software to the file system. The N900 works around space limitations by redirecting software to /opt, but this has to be done by the software developer before release.