Rather than thinking of this bizarre syntax as improper, informal, or simply a regional anomaly, consider its historical origin:
Michael Quinion from World Wide Words, who writes on international English from his own British perspective, traces this verb-subject-object word order back to the 1611 version of the King James Bible. He finds many passages in which a person said something to the tune of "hear ye me" when someone wanted to emphasis a point.
Quinion also notes that the specific phrase of "believe you me" is a rather modern one in America, showing up in writing for the first time in 1919.
What remains unclear is how widespread this usage is currently in the United States. Again, people still use it in Wisconsin to preface an important statement, but where else does this phrase show up? I'm interested to hear your comments...inform you us :)