According to Media Week, the CW will have its presentation at Madison Square Garden Theatre on May 21, 2009.
What are the upfronts?
"In the North American television industry, an upfront is a meeting of television network executives with the press and, more important, with major advertisers. It is so called because it signals the start of an important advertising sales period, allowing marketers to buy commercial airtime "up front", or several months before the season begins.
In the United States, the major broadcast networks' upfronts occur in New York City during the third week of May, the last full week of that month's sweeps period. The networks announce their fall primetime schedules, including tentative launch dates (i.e. fall or midseason) for new programming, which may be "picked up" the week before. The programming announcements themselves are usually augmented with clips from the new series, extravagant musical numbers, comedic scenes, and appearances by network stars, and take place at grand venues such as Radio City Music Hall or Carnegie Hall. Most cable networks present earlier in the spring since they usually program for the summer months; press attention to these announcements is usually much lighter." (source: Wikipedia).
Supernatural was picked by the CW (following the WB and UPN merge) on May 18, 2006, the day of the upfront. The series was renewed for a third season on May 15, 2007 (2 days prior to the CW upfront), and was picked for a fourth season on March 3, 2008, following the writers' strike (the upfront was held on May 13).