"Professional" Social Networking

Many professionals are warming up to the idea of using social networking tools to help extend their professional networks--especially those finding themselves in a job search. Popular tools are LinkedIn and Plaxo, but now that FaceBook has opened up to non-students, this is also emerging with some.

What concerns should professionals have when using these tools?

Should professionals keep separation between their "professional" and "personal" networks?

Should employers search these or other internet sources as they recruit or qualify candidates?