Eric, over at CommonSense PR, has listed 5 key points about choosing a blogging consultant.  Some excellent points there, especially about checking the blogging history of the consultant and ther references.  I would add a further one, in that also check the variety and breadth of previous work.  One thing I have found with digital teams for marketing websites is that sometimes the company can have a very long track record of completed projects but every single one is the ‘same’, they just changed the colours and the words with little or no creative thought.   SO, when choosing a consultant check that they understand the media well enough to know that you cannot apply the same solution each time, each solution has to be created anew.