Some attempts have already been made to port the android on Nokia N900. Till we are not successful, install this beautiful theme to enjoy the android look on your nokia N900.
The theme is very nice and fully customizable. You can also change the wallpapers according to your mood and time. an-Droid theme can be downloaded from the Package Manager. You will have to enable the Extras to find this theme.
Here are some more screenshots of the theme.