Good point, Fran.
However, if MJ is convicted under the Adam Walsh statute, and the statute is held to be unconstitutional on appeal for some reason that overturns MJ's conviction, he can be tried again in the VT courts for the same crime.
The sex-offender registry portion of this act has been ruled unconstitutional by federal Judge Gregory Presnell in Orlando, FL, on April 18, 2008.
http://constitutionalfights.blogspot.com/2008/04/florida-interstate-adam-walsh-act.html
Now, that ruling does not apply anywhere except in the district court in Judge Presnell's jurisdiction. At some point, if the federal appellate courts disagree on the constitutionality of the statute or parts thereof, the issue will go to the Supreme Court for a definitive decision.
I am pointing this out because the status of being convicted under the Adam Walsh Act does not mean it will stick, constitutionally speaking.
It's not double jeopardy, though, to try MJ for the same crime in the VT courts, if his eventual and hoped-for federal conviction is overturned on constitutional grounds, or on any grounds.