Maemo OS is built mostly on Debian. Hence, it is quite possible to run Debian on Maemo based devices. Here’s how: More here
◦The Bundyo-Benson-Build – download here.
◦1.3 GB free on one of your memory cards (this includes the built-in 2GB on the N810). After installation, it goes down to 1 GB.
◦Internet connection on your phone.