N810: 3 OS and 1 DEVICE (Mamona/Maemo/Debian)
I have a N810 and 3 OS in the same device.
Now you will ask me : How?
The answer is below:
1 - Move your Maemo to the MMC internal
* Advantage : you will have more space.
Steps: http://maemo.org/community/wiki/HowTo_EASILY_Boot_From_MMC_card/
Now you have the Maemo in the internal MMC.
2 - Flash Mamona in your flash memory
* Advantage: Nowadays Mamona distro is good for developers. 
Steps: http://dev.openbossa.org/trac/mamona/wiki/Releases/0.1/RunningMamona
3 - Debian XFCE in external MMC
* Advantage: Debian rocks !! 
You should partition your MicroSD in 2 parts:
- partition 1 - 100MB - fat32
- partition 2 - ALL THE REST - ext2
Download the Debian rootfs
wget http://debian.i2c3.com/debian-armel-n800-xfce-beta3.tar.bz2
Untar the Debian rootfs in the second partition:
tar -xvjf debian-armel-n800-xfce-beta3.tar.bz2
Boot using your external MMC.
That’s all!!