WARNING: This guide is experimental and everything you do with this guide is at your own responsibility and risk and will probably void your warranty
WARNING: DO NOT RELY ON WALL CHARGER, ONLY USB CHARGING FOR 1.1 IMAGES, see meego bug #2260
WARNING: If you are unable to boot anything, do NOT let the device stay in u-boot. IMMEDIATELY reflash back to Maemo. Your battery may drain otherwise. You can solve a drain situation by reflashing back to Maemo using another, charged, battery. Or charge your battery in another Nokia device using the same battery type, such as the 5800, using a wall charger. There is a guide to battery drainage solving.
This is a simplified guide for dual-booting with u-boot on Nokia N900.
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. To check this, in Maemo X-Terminal, type "uname -a" and make sure you see:
Linux Nokia-N900 2.6.28-omap1 #1 PREEMPT Fri Aug 6 11:50:00 EEST 2010 armv7l unknown
and "dpkg -l | grep kernel-modules" should show 2.6.28-20103103+0m5.
You will need a 2 GB 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-1.1.0.0.20101101.3-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 the instructions for installing a rootfs on an external MMC card and write the image to your microSD
6. Insert the microSD, put back phone back cover (otherwise, microSD card won't be properly detected at startup by MeeGo) and turn on the N900, let U-boot menu time out and MeeGo will boot.
7. Enjoy!
To boot back in to Maemo turn off your phone, then either remove the microSD and then turn on your phone, or turn it on and interrupt the boot during the countdown by pressing a button and then type "run noloboot".
If you want to remove U-boot you can as root in Maemo run: apt-get install --reinstall kernel kernel-flasher
If you are unable to get back into Maemo, please follow http://wiki.maemo.org/Updating_the_tablet_firmware in order to reflash your device. This will destroy your current Maemo setup, but return your device to a working state
For the technically minded, you can flasher '-F <firmware image> -u' and 'flasher -f zImage -R' to get your original kernel back and hopefully get back to your current Maemo setup. If you have used other kernels than PR1.3 kernel as is required, this will not work.