As others have said, I remember Taker acknowledged it after defeating Flair, I don't remember the first time commentary or WWE acknowledged it though, I just watched his WM20 match vs Kane again and it was acknowledged there and probably before too.
But I don't know if it's true that the Streak was what his WM matches were since his match against Randy Orton. As I recall, his WM match against Orton was about Legend vs Legend Killer, and Orton having killed every legend in WWE and the only he didn't was Undertaker, the biggest legend of them all.
WM22 was just ending the crap feud against Henry and Taker getting his revenge against Henry because he cost him his World title.
WM23 was about Taker FINALLY winning the Rumble and FINALLY becoming champion again.
WM24 was about the amazing feud against Edge and Taker also winning the championship again.
WM25 was about the best of all time vs the other best of all time, and those two putting on the best match of all time.
WM26 was about one of the best of all time, HBK, wanting his rematch against the other best of all time, Taker, in the best match of all time and HBK retirement match.
WM27 was about Legend vs "Legend" (HHH, I put it in quotes because HHH isn't as great as he is billed).
WM28 was about an End of an Era.
WM29 was about ending the Streak, two of the best performers ever wrestling against each other at the biggest stage of them all and the storyline on Paul Bearer (RIP).
Before Taker retires he MUST wrestle against Rock, Lesnar and Cena at WM (and win) and if possible against Jericho, Angle and Sting. And Hogan just to have his crap name on the Wrestlemania victim list.