Actually yes, Mimblewimble according to the HP wiki, is the incantation for the tounge tying curse.
Coincidently, before it was taken down before the site was redone, on Pottermore there was a story that talked about the Dursleys and it said that the tounge tying curse was used by James as a prank the first time he and Vernon met.
I take that scene in SS to be Vernon telling himself to shut up whille he still can.
Update more then a year later: that story is back up on Pottermore but she's edited it. It now says that what completely soured the Dursleys to the Potters was a dinner meeting gone wrong.