Staffing Industry Analysts has a post up suggesting organizations would have better success with contractors if they made a better effort at making them feel welcome:

When contractors are being treated differently from permanent staff, this often leads to low morale and higher attrition rate. With the increase in contract job openings, there will be growing competition for specialist contractors, Bennett said. Organisations should consider offering better benefits, and encourage more interaction and communication with the team to make contractors feel that they are also part of the company.