I've seen some nice and ghetto mods of some using the wintery outdoor air to cool their PC. Personally, I think its a little insane and couldn't stand the thought of condensation occurring on my board causing a short. A sad end to an expensive system.
But since many of you are fearless, you should consider using a window fan to draw air in. It also allows you to reverse and pull air out from your case. That might be helpful during the summer if you have AC and don't mind throwing it out the window, so to speak, to give some extra air flow to the system.
I've seen many mods where the entire window opening isn't used, allowing frigid winter air inside the room. The window fan would at least help block against a nasty winter breeze from passing in. You couuld also have one fan drawing air into the case, and another drawing out.
Just a suggestion.
Also, how do those of you who use outdoor cooling prevent condensation? Or do you just cross your fingers?