Use one of the free Norton Ghost clones for Linux. They're open source, but I don't know if they work on ARM because some are used on Live CDs. There is also 'cloning' which is popular on Nokia N800. I think something similar should be possible on N900 by using MicroSD card.

It'd work like this: one would make an image, give it loan, let them fiddle and play with it. After loan, rewrite image back and nothing has changed. Not even any of owner's settings as it is a 1:1 copy. After this, one can re-loan the device again, or continue to use it themselves.

Alternatively, it could boot from the MicroSD card instead of rewriting the image from MicroSD to flash. Then, it would not touch flash. The other way around takes more work to restore but image on MicroSD you can keep with you; flash not so, so they could access the flash (by mistake or intended). You could also make several images, put them on MicroSD, and have these several images booted depending on user who its loaned out. Saves rewriting images whole time.

All this only works if you trust those you loan the device to though for theoretically speaking once someone has physical access to your hardware they could compromise it for example by installing a bug, cold boot vector, copying your SD card, ...