Dallas-based AT&T Inc. announced that it is terminating an agreement with Motricity, a move that sent stock in the Bellevue, Wash.-based software company reeling. GeekWire reported that Motricity stock fell by more than a third on Monday after AT&T's announcement. The portal agreement with AT&T (NYSE: T) accounted for 42 percent of Motricity's revenue in the first nine months of 2012, GeekWire said. AT&T's MediaNet and ATT.net sites were a part of the portal agreement, GeekWire said. Motricity…