I received the letter, 5/11 build date. I wrote this on Bimmerfest forums:

"From what I can see, the old driveshaft part # 26207597649 was used from 4/10-2/13. This is being replaced by part # 26208605866 which was used in production from 3/13. I'm using an X35d and RealOEM for reference."

Many documented failures some with major damage.

Good reading here: 2011 X5 Driveshaft fail - Bimmerfest - BMW Forums