Unity is a commercial company, so they will go where the money is. If they see that the N900/Maemo platform is a big market, and/or if their customers (i.e. the developers that licence their engine) ask for them to support it, I expect they will.

As end-users, I guess we would have to ask developers of our favourite iPhone games if they can port to N900. I'm not sure if it's worth end-users contacting Unity directly, as we aren't their customers.