I will never trust Ricky Schroder.... but having him be the mole is too convenient. So who does that leave? Bill Buchanon, Milo, Chloe, and Miles.
I seriously doubt its Bill Buchanon so I'll scratch him off the list. I don't think it will be Miles since then his mental brakedown as a result of helping the terrorists arm the bomb would not make sense. This onle leaves two choices Chloe and Miles. Since I don't see Chloe as a fem fatale I'll have to say... that it was Milo. Why? Remember ealier in the day when Nadia was unable to log into her computer? Milo gave her his account which unfortunately might mean that when Chloe finishes her back hack it will say that Milo was guilty, BUT since he was missing for an hour or so during the mission with Grae Bauer's wife he could easily say that he wasn't able to set up any of the backdoors which the terrorists used to monitor ctu. BUT, these back doors could actually been installed onto his log in name so Nadia will end up taking the fall for him until later in the day when the "mole" becomes active once more.
Now... an other good question is... who killed Audrey Rains? Assuming that the murder has appeared thus far, I'll vote for Kim Bauer (even though it most likely one of Jack's dad's henchmen or a Chinese villian who appears either next season or in the movie). The only reason why I'm saying Kim Bauer is because KJack killing her would be poetic.