Rotoworld [1] lets us know that Rams wide receiver Donnie Avery [2] has a sprained foot that'll keep him out for 4-6 weeks.
They note that sprained feet tend to linger; a good example would be Arnaz Battle [3] last year, who sprained his foot in Week 9 and never made it back onto the field. Deion Branch [4] sprained his foot in Week 5 of the 2007 season in a game against the Steelers and missed five weeks; he averaged 86 yards per game in the four games before the injury (110 if you don't include the Steelers game that he was injured in), but only 53 yards per game in the six games he participated in after his return.
Links:
[1] http://www.rotoworld.com
[2] http://www.footballoutsiders.com/player/15348/donnie-avery
[3] http://www.footballoutsiders.com/player/15395/arnaz-battle
[4] http://www.footballoutsiders.com/player/15474/deion-branch