My vote was for it being close to when the chip hits the felt. The point of the rule (I think) is to prevent a player from making his decision based on the reaction of other players. If it's close (and I mean real close... less than a second) to the time when the chip hit the felt, the verbal declaration should hold. It's not enough time for a player to have a reaction, much less to decipher the reaction. If a case can be made that elite players (or those who are really good at reading faces) could, in fact, read the reaction of other players in that short time, then I would change my vote to the chip hitting the felt.
PS I am assuming that when you say "close" you are referring to after the chip has hit the felt.