It would be interesting to know whether San Diego designated Whitehurst as their designated third quarterback last week.
A team is allowed to dress a third quarterback and not have him count toward the 45-man roster, so long as he's designated the third quarterback. If your designated third quarterback plays in the game at any time before the fourth quarter, however, you may NOT bring back quarterbacks no. 1 or no. 2.
If the Chargers DID designate Whitehurst last week (or if they routinely did during the year) but do not designate him this week, it tells you that Rivers is not likely to play much. If your number 1 is seriously hurt, you really don't have a choice except to make your number 3 a part of the 45-man roster. Otherwise, you're taking a tremendous gamble. If Volek has to sit out even one play in the first three quarters -- equipment, gets his bell rung, wind knocked out -- the only way Whitehurst can play and later come out of the game is if he's on the 45-man roster.
I guess if I were the Chargers I'd figure that if Whitehurst has to come in before the fourth quarter we're screwed anyway, so might as well not take up the roster spot. Pretty big gamble.
I don't know when teams have to designate the 3d QB or how to find out, but it would be somewhat useful information to have. I imagine it happens on game day.