In PA it's legal. I know in VA / DC it's not. I'm looking to get one too. The best detector I ever had is a Belltronics Vector 945...That thing "sees" 'em from miles out!!
Edited with this info from Bell's website:
Radar detectors are completely legal to use in non-commercial vehicles in 49 states. Virginia and Washington DC are the only places in the United States that currently ban radar detectors. Radar detector use in commercial vehicles has been prohibited in the U.S. since 1995 and radar detectors are also illegal in most Canadian provinces. We do not track detector regulations in other countries.