O.K., I know we won't know this yet, but I think this would be a fun topic to discuss anyway. In the the first game, Triss was talking to a sorceress (Phillipa?) in the mirror and there was mention of a truth that's being kept from Geralt, and this was also brought up at the end of TW2 when Phillipa told Geralt that Triss was innocent of the assassination plot but worded it as "innocent of this" which implies she's guilty of something else. And if you walk away from her to save Triss, she yells as you walk away "So you think she's innocent!" In the first game, I was pretty certain that it was knowledge of Yennefer and his relationship to her that Triss was hiding from him, but now that Geralt knows about Yennefer and is understanding she was very special to him, this reason doesn't have as much weight anymore as well as being too obvious. I mean, for this to be brought up in two games tells me that this could be one hell of a gut punch when the truth is finally revealed.

She's a sorceress. She's guilty of lots of things, as they're all manipulative hotties. ;p

Whether she's guilty of something directly related to Geralt in a meaningful way....I don't know. The first thing that would spring to mind is anything that related to his gaining amnesia, as she was definitely the biggest benefactor of that after he forgot about Yen, but it seems fairly clear that she couldn't have had anything to do with that.

Maybe we'll be surprised with some shocking plot reveal concerning Triss in W3, but it could just have been a blanket statement that would cover the general manipulative nature of sorceresses.

It has been a while, but I seem to remember something similar stated in The Witcher 1, after Triss rescues you after the battle at the Swamp Tower. When Geralt is recoving in bed and listens in on the conversation between Triss and the enchanted mirror, I'm almost postive Phillipa says something that suggest that Triss is hiding something from him.

That one is easy. I'm almost certain it's about Yennefer. Even in the beginning of the game, she coyly doesn't mention Yennefer, when Geralt has this hazy idea that he'd been in love with a sorceress before, leaving open the suggestion that it was her instead. ;p

Thing Triss is hiding is that she is actually your mother and you're your own father, hence your sterility or else you'd end up making yourself in triplicate. :D