Take a look under there and you should be able to see what the problem is. The tech may have left the clamp loose (its a 17mm nut IIRC), the exhaust gasket may have blown out, or your header pipe is broken. Either one should be easy to see.
I'm guessing this has to do with the exhaust gasket - the same one that was caused a recall for most GTS in 2007. I did get this new gasket put on before my trip in Keene, NH. It has been working fine for over 2,600 miles. I'm guessing the guy at Vespa Idaho Falls didn't put on the gasket correctly?