I think the discussion about whether blogging should count for tenure in the US amounts to little more than wishful thinking. To consider it anything "like" equivalent to refereed publications is not realistic. No matter how many hits a site might get, no matter how much discussion might be generated or feedback given, blogging is not only outside peer review, but it is outside the guild. Not only can the blogger write anything she or he wants, but anyone can respond in any way. This is not peer review and it is not critique by the guild. It is not even close to it.
I have published scads of articles and books that have gone through peer review, and it is not an equivalent process to blogging and responses from other bloggers. Whether or not you think that the anonymous peer review process is "elitist" or "traditional," it is what makes what we do a profession, controlled by a community of professors who have had years of training and uphold rigorous and necessary standards. Without it, we would become editorialists and teachers, not professors.
What about blogging and service? My experience with the tenure process is that service means service to your university and your department. There are some overtures to public dissemination of the guild's knowledge, but if this appears to be too generous on the part of the person seeking tenure, it can work to her or his disadvantage. The question will come up, why has this person spent so much time outside the guild? How much more could this person have published if only she or he had spent the time doing that instead of public service? So what "service" really means is "committee work" with an occasional public lecture.
I have real doubts about blogging being a good thing for a tenure case. In fact, if it were me, I would tend to downplay it. When my dean found out that I had begun blogging, his reaction was sincere worry. He wanted to know how I would continue with my writing-for-publication agenda if I spent time blogging. And I am a full professor with tenure and a chair!
Even though each university has its tenure quirks, I really put out a caution to those of you who are in your probationary period no matter your university. I don't suggest stopping your blog, but I do suggest that you make sure it does not interfere with writing-for-publication or committee work. Blogging is not the same as publication or service. It will not be considered the same no matter what arguments you try to make in your tenure document. And if you emphasize it too much, it could raise some eyebrows.
Update 4-14-07: some interesting discussion on other people's blogs