When you should write the GMAT test..
Or, to put it in a more desi e-style, "when to give GMAT?", a simple answer to which is "whenever you are ready!". All news that you hear about the GMAT software getting upgraded in June, is a myth. Even if there were any element of truth in it, there really isn't anything one can do about it, except for dealing with the question that flashes in front of you. Just write the test when you feel you are ready.
If you're fresh out of your college, and (for some reason) want to do an MBA later, take the test soon after you finish your college. The timing is perfect as you would have sufficient time for your preparation. The score stays valid for 5 years.
If you are experienced, you can take the test in June-July of the year before you'd like your MBA classes to start. That means, if you are aiming for Fall 2010 intake, take the GMAT test in June-July 2009. Taking the test early would also give you an option to retake the test, if you're dissatisfied with your score. The round 1 application process has deadlines around Oct-Nov, and you can retake the test in Aug-Sept (if needed). This leaves some time for your introspection (essays).