Bill,
Its marketing to realitors. Its a proven fact that someone will buy a nice house thats ready to move in, and CLEAN, rather than a trashed house. Atleast in the high end market which most of us specify in.
Turning and flipping houses is a totally different story. Larry(meAt) would be a good one to ask about how to market to them, or that service. Seems hes got that niche down well.
When I have done this in the past, I would even do little things like wiping baseboards and window sills off as a nice courtesy. But always mentioned I went that extra mile for the realitors client. They do appreciate the little things. It shows attention to details, and that you care.