After lots’s of tries and help from different people I got my own kernel running on the N900, which is based on the Debian kernel package. So far it just offers basic features (e.g. framebuffer, keyboard) and is missing some important ones (e.g. touchscreen).
To test it on your phone you need to do have Debian installed on an µSD card, install uboot on the N900 (as described in Debian on N900: U-Boot) and install the kernel package.
You can fetch the package from http://pkg-n900.alioth.debian.org. I also uploaded the kernel config next to the package, so that you can check if a specific feature is enabled.
I will ask for inclusion into the official package now.