The two Pilatus accidents - Butte Montana March 2009, and Bellefronte Pennsylvania March 2005 have something in common with the Buffalo New York Bombardier -8 accident January 2009, besides icing conditions. Perhaps the T-tail configuration is more susceptable to tail-stall with icing.
THe NTSB probable cause for the PA PC-12 accident is : "The pilot's failure to maintain sufficient airspeed to avoid a stall during an instrument final approach to land, which resulted in an inadvertent stall/spin." Icing is not mentioned. More recent commentary on the Buffalo accident is similar in content. IMO, the probable cause for the Butte accident will be substantially the same.
I attended a briefing by Pilatus engineering personnel on the known icing certification program for the PC-12 a couple of years ago. It was an eye opener in terms of the comprehensiveness of the test program. Probably the most thoroughly evaluated FIKI certification for an acft of this class to that time. While it is remotely possible that ice was a factor at Butte, IMO it is the least likely of the scenarios put forward to date.