Yes, we can! ruby on rails works well on nokia n900.
Install ruby from maemo repository (dev).
Download and install rubygems (use –no-rdoc and –no-ri).

Rubygems creates only /usr/bin/gem1.8 so you should create a soft link like this: ln -s /usr/bin/gem1.8 /usr/bin/gem

Type gem install rails -V and wait.

Now you are ready to start your rails app on nokia n900!
Just run ruby script/server in your project folder. To create a project folder use rails my_app as usual.

Your server may not start if you miss libopenssl-ruby. In this case you get an error about net/https: apt-get install libopenssl-ruby.