This is certainly an issue at the high stakes scene, and there is really not much, poker sites can do about it. People will usually only get exposed, if they do something stupid like telling friends, that they are cheating.
Yesterday I had been working off the tables with ICMizer and forgot to close the program become opening up the PokerStars client. Right away I had a warning email for breaking the TOS, even though I had not yet played a single hand. The email is system generated and tell, that PokerStars know, I probably did not do this on purpose, but if I do it again, they might take "sterner action", which I guess mean, they will steal my money.
The problem though is, that they are only cracking down on people like me, who dont actually cheat, but just do, what PC users have always done, which is to have programs running, even though you are not using them right now. The actual cheaters of course have ICMizer or their solver running at another computer, and there is no way for PokerStars to ever figure that out.