My Buy More mole just tipped me off that Josh Schwartz is casting younger versions of Chuck and Ellie ( 7 and 11, respectively) to appear in a flashback sequence in Chuck's season finale. No other details were provided, but I spy a connection between this and Scott Bakula's return as Bartowski patriarch Steve. Also, if you recall, Schwartz recently declared that Bakula's Chuck comeback arc would be the biggest thing we've ever done on the show. Put it all together and what do you get? A comments section bursting with theories. (That's your cue to tell me what the hell it all means.)
UPDATE: I'm told the ep will flash back to the night Pop B left home, forcing Chuck and Ellie to become each other's family.