You can retake the test, but you have to wait a year from the date of your last exam. Let's say, for example, that you last took the PMP exam (for the 3rd time) on November 4th, 2010, you will have to wait until November 4th, 2011 to reapply. Reapply is different than take the test, because when you reapply you have to wait the usual wait to get accepted and schedule the exam.
In short, you have to wait around 13 months from the last failed test to TAKE the test again.