MBTI is a measure of you
preference. E/I, S/N, F/T, J/P are all preferences. Of course you can switch to the other side. What happens when introverts need to speak publicly? Or when extroverts need that moment of silence to think things through? This test is essentially a filter for your natural inclinations.
Not being relaxed or in a stable state of mind may affect the test's outcome. If you are stressed about a project or super pumped from your trip to Europe, your test will give different results.
Then there is the pop culture influence. Some people may not answer a question correctly because the answer isn't socially sound or in sync with their impression of themselves.
Though online tests are OK to start with; the results should be verified by talking to an MBTI practitioner.
I personally find this MBTI system very helpful in studying myself and people around me. But its true, you need to read up on it before you can start making comments about the test and the theory behind it. It has a lot more intricacies than meet the eye.
It would be very interesting to see the makeup of HNers. Go take the test at http://www.humanmetrics.com/cgi-win/jtypes2.asp then read up the description on http://www.personalitypage.com/html/portraits.html. See if it matches with your impressions of self. If not, try to switch one of the letters and read up that description.