During the game today, Brett Favre left the game during the 3rd period after a hit from Giant's Strahan and William Joesph.
Source: http://www.nfl.com/teams/story/GB/7759451
Packers lost 14-7. The game winning score was a 4 yard pass from Warner to Shockey in the 4th period.
Update: Packer's Head Coach/GM Mike Sherman says that Favre is "ok". Favre won't practice until Tuesday.
source: http://sports.espn.go.com/nfl/news/story?id=1894843