"FOR GONDOR!" (Gondor can be replaced with any semi-nerdy referance, however, fully nerdy referances do not work for this one to have full effect as you get "what?"s rather than nervous laughs.)
When I played WoW, on WoW:
My RL friend and leveling partner: Charge forth!
Me: For Gnomeregan!
(This one made me wish WoW would drop the language barrier, as I suspect nothing would infuriate Horde players more than a pair of Gnome-Nationalist RPers with a bad sense of humour and little desire to actually win)
"GOTWOG!"