Stay or Go? Remaining in the same lab as a postdoc

A very simple question was posted over at the NatureJobs forum recently and to my eye the responses are quite disappointing so far. It deals with the not-uncommon scenario whereby a doctoral student successfully defends his or her dissertation and then spends additional time in the same laboratory as a postdoc.
The reasons are many and varied. Perhaps there is a paper in the process which requires additional work, either at the bench or simply in revising. Maybe the person simply hasn’t landed precisely the right “real” postdoc and wants to take a little more time to find a position. It would not be unusual that the person is in a relationship with another professional who cannot leave town immediately.
The question, however, deals with perceptions.

Do employers look favourably or unfavourably on Ph.D students who stay on for some time (6 months to a year) as a postdoc in the same group?

What do you think DearReader? Is this rare? Common? Tolerated? Does it put a permanent black mark on your CV? Does it matter only minimally compared to the upside of getting the paper from graduate work published?
What think you?