Rumor has it this is the official date. More as this develops.
UPDATE:
Will Shipley, original author of this tweet, claims that “there are tons of clues in the newest version of Portal.” These are: GlaDOS version 3.11, March 11th being during GDC and Gabe Newell getting the Pioneer award that day. (tweet)
UPDATE #2
After spending some time on the official Steam forums it turns out Valve released a lot of hints pointing towards the 3/11 date and the “Pioneer” awards, including references to the Apollo space program (and hence the Pioneer Plaque ). See some of the forums posts here and here.
Waiting for further proofs.