The problem, explains Secunia, is that Blackberry, made by Research In Motion, "doesn't handle meeting request messages with a string larger than 128KB in the 'Location' field." It says that "this causes the watchdog timer to reset the device".
According to RIM, this doesn't cause any loss of data. The problem only affects version 3.7 Service Pack 1 and some earlier versions. RIM has fixed the problem in version 3.8 of the handheld software.