On Boxes and Arrows, Andrea Wiggins wrote an article a couple of months ago that I somehow missed on using Web analytics data during the persona creation process. In her example, a designer uses Google Analytics to provide real behavioral data for each of the segments or personas that she hardwires into the analytics tool. I absolutely agree with Andrea that behavioral data is critical to a well-rounded portrait of users. It's not enough to just talk with users and hear what they think - watching them (whether qualitatively through field studies or usability testing or quantitatively through Web analytics) is equally important. I like how she extracts data from Web analytics reports to make the personas more real.
The challenge is that the quantitative data could be placed on top of incorrect qualitative personas right from the start. If the personas you invent aren't right, no amount of data on top will help. That's why I'm a fan of the quantitative persona creation process, where data from surveys and Web analytics gets used via statistical analysis to generate the persona segmentation in the first place.
Make sure to read the good discussion after her article.