Recruitment and Retention

There seems to be a lot of interest in recruitment practices for Agile teams, but I have heard much less about the retention practices thereafter. This is perhaps understandable given that recruitment is explicit and visible, and retention is not (both in terms of the time and money required). But retaining skilled Agile staff is just as important as recruiting them in the first place, and is probably harder to do*.
So how much attention do Agile organisations need to give to retention? Obviously it becomes a higher priority when the job market is buoyant (as it has become in the UK in the last six months), if only because of the personnel agencies whose financial wellbeing depends on persuading staff to change jobs fairly frequently. But in general what are the factors to consider?
Well, Agile team members seem to be happier on the whole (i.e. less likely to leave), and Agile teams are certainly less dependent on any single person because of the way that knowledge is shared around. But on the other hand, because Agile teams can be so "close knit", one person leaving an Agile team can create a “diaspora” effect: others in the team start to change their employer not because the present one is especially bad, but because they value continuing to work with the people they work well with.
Perhaps there is a real financial risk to Agile projects that pay only lip service to retention, whose team members will be at least as loyal to each other as they to are to a employer?
Afterword: this post was stimulated by the rising number of ex-ThoughtWorks staff now working at the same London investment bank. (And yes, I am one of them.)

* I say retention is harder because it requires a continual effort to understand people as individuals – what they find motivating or threatening, what growth paths are open to them and so on, all of which will change – and an organisational structure that is adaptable enough to meet (and keep on meeting) a wide range of sometimes contradictory wants and needs. Whereas recruitment (even recruitment for Agile teams) is often reduced to a process that requires little or no organisational commitment.