"Personas are archetypal users of an intranet or website that represent the needs of larger groups of users, in terms of their goals and personal characteristics. They act as ‘stand-ins’ for real users and help guide decisions about functionality and design."
I'm seeing a lot of posts about user task analysis, user path analysis, user centered design.... but nothing about personas.
No, a good persona won't replace usability testing, but used correctly, they should help make usability testing less painful.
And unlike more usability testing, referring back to your persona is free.
Personas can be highly valuable in keeping your graphics designers, content editors, and IA structure folks on target as your site grows and changes.
(quote is from http://www.steptwo.com.au/papers/kmc_personas/index.html although wikipedia has more, too)