There's no requirement on when the annual review must be done, so technically you could wait until September 2010 and review the last 12 months. If there were less than 5 reportable transactions, you would revoke the exemption right then (no need to send in revocation anymore).
I like to review all of my exemptions during the same time each year around March. So if I exempted a customer in September '09, I would look at them again in March '10, looking at the past 12 months, and if they don't meet the eligibility requirements, I would revoke them right then.
No matter when you do your annual review, just make sure that you do it at least annually, you're looking at the most recent 12 months, and you're revoking them right away if necessary. I'm not sure about your reference to waiting until March to revoke. You can and should do that any time. Again, you don't have to send anything in anymore. Just update your internal lists and start filing CTR's.
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Don't make me say, "I told you so!" Sincerely, your friendly Compliance Officer.