Yes it's true there's going to be a Simpsons and Family Guy crossover episode called "The Simpsons Guy" that will air in September 2014 (there's also going to be a Futurama/Simpsons episode too!).
Peter (Seth MacFarlane) and the Griffins get out of dodge and end up in Springfield, where they are greeted by a friendly stranger named Homer Simpson (guest voice Dan Castellaneta), who welcomes his new “albino” friends with open arms. The families get along famously: Stewie (MacFarlane) becomes obsessed with Bart (guest voice Nancy Cartwright) and his old-fashioned pranks; Lisa (guest voice Yeardley Smith) takes Meg (Mila Kunis) under her wing and is determined to find something – anything – at which she excels; Marge (guest voice Julie Kavner) and Lois (Alex Borstein) ditch housework for a little bonding; and Peter and Homer fight over the best beer in town – Pawtucket vs Duff.
And here are the first pics: