Just to resume a little bit, so you don't have to read through the thread, if you're new but interested.

We would like to make it possible to "backup" the entire operating system, inclusive every program installed, every preference made and every modification made.
This process can be compared to the debian-script "mklivecd" which is used to make a bootable live cd from your current system.
The aim is then to provide different ROMs which are modded, for example a ROM made to speed up the system as much as possible (OC, transitions removed, maybe different browser and so on) or a ROM for low as possible battery usage (UV, automatic disconnect and so on). Also it will be very easy to backup your system in the current state, so you don't have to reinstall everything after messing up your system and reflashing it. Just flash the last working state.
Currently we have four options. In my opinion they promise success in this order:

1. Use USB recover mode to create an image and flash this image afterwards (needs a PC to create and flash the image; dualboot should be possible)
2. Use the 0pen Free Fiasco Firmware Flasher (adding N900 support required)
3. Port mklivecd to maemo (maemo is debian-based, so it shouldn't be difficult)
4. Try to profit from the backup function of Familiar Linux for the iPaq PDA (major work required)

Thanks to rash.m2k, santiago and abill_uk for their time and effort.