WARNING: This guide is experimental and everything you do with this guide is at your own responsibility and risk
This is a simplified guide for dual-booting with u-boot on Nokia N900.
0. You must be using the PR1.3 kernel and nothing else. You will likely end up in a reboot loop if you don't have this otherwise. You will need a 2gb microSD card or bigger.
1. Enable the maemo.org extras-devel repository, see http://wiki.maemo.org/Extras-devel (The software hosted in extras-devel is not ready for normal users! PLEASE PLEASE PLEASE don't play with it unless you really know what you are doing. It is recommended to disable the extras-devel repository after you have installed the package below, and do not install any updated packages that you might be notified of while extras-devel is enabled. Be ready to file proper bug reports instead of posting complaints.)
2. Install the 'uboot-pr13' package from 'System' section
3. Verify it's installed by restarting your N900. A penguin should show up at startup and U-boot will be mentioned, let it time out and boot into Maemo.
4. From http://tablets-dev.nokia.com/meego-codedrop.php (type in your IMEI), download meego-handset-armv7l-n900-final-mod-184.108.40.206.20101101.3-mmcblk0p.raw.bz2 -- we require IMEI/EULA but in practice the images are redistributable under certain conditions, see the license files for more information.
5. Follow http://wiki.meego.com/ARM/N900/Install/MMC#Installing_Rootfs_on_external_MMC_card and write the image to your microSD
6. Now, when the microSD is inserted and the Nokia N900 is turned on, MeeGo will boot. Otherwise it will boot into Maemo.
8. In case you want to uninstall u-boot, apt-get install --reinstall kernel-flasher