A Family Guy movie? No thanks. I think cutaway gags and random 70s and 80s pop culture references for 90 minutes would start to grate on the nerves. Also, like the Looney Tunes characters and Tom & Jerry, the FG characters simply don't have deep enough personalities to sustain an audiences' attention for over an hour. Even the Stewie Griffin: the Untold Story DVD was just 3 strung together TV episodes with a continuing theme.
I give Seth McFarlane some credit for not wanting to do another "main characters save the world" plot, but what else could an FG movie be about? Like others have already said, what could they do in the movie that hasn't already been done on the show? We've seen just about every aspect of the main characters' lives, past, present and future. Frances Griffin has died. Peter has met his real father. The Cleavelands have divorced.
Also, like Jazman said, it would be best for Seth and company to wait before pursuing such a project, otherwise they'll just be hearing "The Simpsons already did it!" about 30 or 40 times.