There are at least two ways to pronounce the word "data". How was the pronunciation of Data's name in TNG decided?
Patrick Stewart crystallized the pronunciation during the first script reading
I'm at the Star Trek 50th Anniversary Celebration in Las Vegas right now. Brent Spiner just revealed the answer to this on stage.
He mentioned that he himself would have pronounced it "Daa-ta", but Patrick Stewart read it as "Mr. Day-ta" in the first script reading (in front of Gene Roddenberry) and it stuck.
Spiner added that he believes Patrick Stewart's pronunciation of the word has affected its overall pronunciation in North America.