I don't know specifically about United States laws about public videotaping,
however Walt Disney World here in Florida does considerable promotion video-
taping around the various resorts for use in advertising, Disney Channel shows,
and similar causes.  They post large signs wherever outdoor general taping is
going on which say that:
 
This area is being filmed for promotional purposes.  By entering this area,
you are granting permission for the use of your image in the video productions.
 
(or similar language)....these are used to (1) alert people that an area is
open for them to walk in or through, but that videotaping is going on, and
(2) it saves a lot of paperwork in getting releases.
---Dean DeBolt, University of West Florida, Pensacola