WARNING: This guide is experimental and everything you do with this guide is at your own responsibility and risk.
WARNING: DO NOT RELY ON WALL CHARGER, ONLY USB CHARGING FOR 1.1 IMAGES, see meego bug #2260. (no longer applies to images from april 2011 - wall charger is better.)
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. If you aren't, you will likely end up in a reboot loop. To check this, in Settings -> About product. Make sure you see:
You will need a 4 GB microSD card or bigger. You may also see "MMC", which is basically the same thing as the microSD card.
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 typing "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 the required PR1.3 kernel, this will not work.
This is riskier, so please use the previous way instead.
Note all warnings in the previous section.
You must be using the PR1.3 kernel and nothing else. If you aren't, you will likely end up in a reboot loop. You need dpkg tool to use this.