U-Boot installing:

• Add U-Boot to power kernel v47 using this guide: http://talk.maemo.org/showpost.php?p=876169&postcount=1
  I can supply a prebuild file if someone wants it (and have a public area where drop it)
• Save the obtained kernel file in /boot/multiboot with name vmlinuz-2.6.28.10-power47 (replacing existing file that you must have if you are already using multiboot with power kernel)
• Reboot to Maemo with original kernel or to Nitdroid
• Reboot to Maemo with power kernel.
  This will reflash kernel and so will activate/install u-boot

You will have U-Boot only when you will be under power kernel.
Off Couse U-Boot can be added to Maemo's Original kernel and to Nitroid's Kernel too.

How to boot to MeeGo:

• Download MeeGo's Disk Image (kernel file is not needed) and write it to MicroSD, off course if you don't have already a prepared MicroSD with MeeGo
• If you are under Nitdroid's or under Maemo's original kernel, reboot your phone and choose Power kernel
• Shutdown your phone under power kernel
• Power On your N900 with keyboard opened

How to show Multiboot's Menu (i.e. to choose with which OS boot):

• Shutdown or Reboot your phone
• If you rebooted/shutdown under Nitdroid's or under Maemo's original kernel, power on your N900 with keyboard opened.
  Otherwise (u-boot power kernel) keep the keyboard closed to prevent MeeGo autostart
• If U-Boot appears wait until it writes "loading kernel image ... OK" and then immediately open the keyboard to activate/show multiboot's menu before booting sequence can start