'Nightly News' Going HD (http://www.tvweek.com/page.cms?pageId=578)
NBC's to Be First National Newscast in Format
"NBC Nightly News" is about to become the first national evening news broadcast to go hi-def. The show will begin broadcasting in the format in March; an exact date is not yet decided.Full Story> (http://www.tvweek.com/page.cms?pageId=578)

Looks like last week of march is the date....by the press release
sorry i lost the link

NBC Nightly News to Broadcast in High Definition (http://www.movieweb.com/tv/news/43/18543.php)
MovieWeb - USA
NBC News will be the first news organization to offer its network evening newscast, NBC Nightly News in High Definition (HD) beginning on March 26, 2007, ...