<p>I am interested in using/learning RoR in a project where I have to use a .NET dll. Is Ruby capable of importing a .NET dll?</p>

If you use IronRuby (Ruby implementation built on .Net) then it should be able to.  If you're already using .Net and want to try out Ruby then you might want to look into IronRuby.
Outside of IronRuby I'm not sure.  I haven't used Ruby myself, so I don't know what kinds of things it's capable of.