From the New York Times:
[S]uddenly, there it was this week, the exclamation point. No longer was it “Forward.” It was “Forward!” on placards and banners at Mr. Obama’s stops during a two-day, round-the-clock swing through eight states, the newly punctuated slogan made its debut. The campaign called no special attention to the change, made no announcement, and yet there it was.
As of now, however, the Obama campaign website is still using "Forward." with a period.