A member of Rep.  Denny Rehberg's staff who suffered a severe head injury in a Flathead Lake boat crash last summer has resigned, Rehberg said Monday.  Dustin Frost resigned as Rehberg's state director to pursue a private consulting business in Billings.  "When someone as intelligent, self-motivated and hard working as Dustin leaves, there's always a hole to fill," Rehberg said.  "Dustin wasn't only my state director, he's a good friend."  Frost said he has always wanted to own his own business.  "The last six years of my life have been more exciting and fulfilling than I could have ever imagined," Frost said.  "I am grateful to have had the opportunity to serve on Denny's behalf and leaving was a very hard decision.  But I'm excited to face new challenges in the private sector."