Not sure whether this is a Mango-specific bug or not, but it is a definite (epically bad) bug with Verizon Trophy.
The Trophy is a world phone, and travels OUT of the US with no problem. However, upon return TO the US, the only way to restore US data service is a full hard reset. See related thread in the Trophy forum (I do not know how to link to it here -- my apologies!). I am running RTM Mango, with no sign of update yet from Verizon, and no success with the infamous 'force' method. Several others who posted in the other thread had identical problem: no data at all, no 3G, no nothing upon return to US, until hard reset. Support could not explain and could not find a solution.
Have other Windows phone models (non-Trophy) had this problem? If so, perhaps a Microsoft bug... if not, perhaps a VZW-specific bug. Either way, a world phone that cannot make international transition in both directions is a pretty big failure as a world phone.