There is solid proof. And... well, I know why Nonye's acting hissy over this. xD It's typical SasuSaku fan behavior, I've encountered it a lot of times already. xD It's because Karin is the closest girl to Sasuke now, and he doesn't care anymore about his recent past (i.e. Team Seven). And thus, after this, she might get defensive on my a**. xD

Anyway, as I said, solid proof, when he was talking to Suigetsu that he knew them for long.
QUOTE(Uchiha Sasuke @ Naruto chapter 348, page 2)
I've been planning for this ever since I came to Orochimaru.
I picked you all out a long time ago.
And then, there's Karin's side.
QUOTE(Karin @ Naruto chapter 347, page 17)
"!" This chakra... it couldn't be...
Shows she knows Suigetsu and Sasuke.
QUOTE(Karin @ Naruto chapter 348, page 6)
So it's you guys after all...
Shows she's
familar with them.
QUOTE(Karin @ Naruto chapter 363, page 15)
Weakling! You're even the man that beat Orochimaru!
QUOTE(Uchiha Sasuke)
Orochimaru was already weak. That's all there is to it...
And yeah, plus the fact that Karin wants to keep some of her secrets. Maybe she did an experiment on Sasuke and would be able to bring out the sleeping Orochimaru in him. Or that maybe Sasuke offered her something to go over to his side, seeing that she's a fangirl, who knows. And yes, I'm hinting at something lewd.
Sasuke listens to Karin. Yes, he does know that she's loyal to Orochimaru, but he also knows that she's important. He asks for her advice on things like he trusts her, and that's a big deal seeing that he's traitorous himself (aw, he backstabbed Orochimaru, damn it).
Actually... although I do like SasuSaku... maybe I'll convert to SasukexKarin. >_> Sounds more awesome, someone able to control the Emo Boy, lol. She actually defies him.