For the N900 just install the UKeyboard package
http://maemo.org/downloads/product/Maemo5/ukeyboard/

it includes a Khmer keyboard layout and should also automatically download and install Khmer fonts.

For Android you will probably have to root the phone just to install a Khmer font (I think you have to overwrite one of the Android system fonts and give your Khmer font the same name to get it to work) - Even then the Khmer letters probably won't form properly because Android phones do not yet have proper complex script rendering support. (Wait another year and this may change)

good luck