You maintain Diamond status by either staying 28 times/60 nights or spending at least $40,000 with the Amex Surpass card. Either one will grant you Diamond status for the remainder of the year and through the next year.
If your Diamond status expires March 2012, then you will need to stay at least 28 times/60 nights during 2011 or spend $40K+ in 2011 to continue status after March 2012.