I would be remiss if I did not call attention to a fun little web app that has hit the blogosphere lately. Rather than analyze a person in terms of Type, Typealyzer analyzes one's blog's Type. I don't know what algorithms it's using to generate the results, or how many blog posts it scans. I've seen mention that it only scans the most recent blog post, which may vastly skew the results. What's neat about this is that it doesn't attempt to analyze the author's Type, but their blog's Type, which can be very different. It's a good illustration of when one's Operating Type is different from one's Best Fit Type - we all visit various Type "zip codes" throughout our day and our life, but always return home.
I wouldn't be surprised if the basis is "frequently used words" by certain types, weighted according to preferences. There's a Type table in MBTI literature that lists words preferred by certain types (with appropriate overlap where Type overlaps).
What's important about this is the application of such analysis - if you're doing a PR blog, you're going to want to use more Extraverted Feeling vocabulary, as societal expectations are the basis for most Public Relations. If you're doing a blog that's more like a personal journal, and there's some differences, it reveals where one is visiting during the writing process (which overall is an Introverted activity, as opposed to the Extraverted preference for talking). Knowing that the writing process is going to be read by anyone changes it as well, therefore making blogging not a truly Introverted process (particularly since the editing and self-critical steps of writing are skipped).
Overall, it's a fun app. This blog comes out as ISTJ by the way. Not surprising given the number of lists, rules, and tables present. I also have to step WAY out of Type knowing that the results will be published and reviewed (I am an ENFP for the most part, and my writing style is nothing like my dynamic and interactive speaking style).
So go have fun with it.