Blogging may be good for you.
Read the results of a three-year-old survey (I know, I know) here.
The survey has its flaws, not the least of which is an infinitesimally small response rate coupled with a lack of data about how the respondents might differ from the blogosphere as a whole, but it is interesting nevertheless.
The conclusions, and what others make of the results, do not mean that blogging is actually therapeutic--for that, you'd need some data other than self-report on whether blogging produces change. A literature search of the databases available through the American Psychological Association's website produced 107 hits--and not one empirical study on the therapeutic value of blogging. One fellow wrote an article referencing this survey and calling for research on this potentially fascinating topic, but nobody's responded yet.
All the survey tells us is that therapy is what many people hope for when they blog, and that they feel, afterwards, that it was helpful. This kind of self-report data is highly prone to bias. And there was an article in the New York Times in April questioning whether "name-brand" bloggers were actuallly stressed by the pressure to post, and post well. This refers specifically to information workers, however, not the average blogger.
There is a good bit of research out there on paper-based journaling that does show, especially if (a) it's problem-solving oriented in nature rather than just complaining, and (b) entries are processed in therapy or in a group, that it can be quite helpful. I believe that these results ought to generalize quite nicely to blogging. Studies of blogging following the design of journal-therapy studies would tell us.