I second what PatrickBateman says, at least for where I'm working at.
You sign a (10+ pages) confidentiality agreement when you're hired.
There is a register maintained by every MC on which everyone who has access to specific information about a client is included. Your position in the register is updated monthly, listing confidential information you have access to. You get a monthly e-mail telling you your position and which clients you are involved with (obviously there's a time frame where the client is still listed though you are no longer staffed on it). You can't buy or sell stocks of those companies as long as you are associated with them in the register.
As someone said, it is fairly unforceable as long as you don't trade big time (but I would not do it either). The very thing I would be careful of is speaking about the client with people not on your team. At MCs, you don't have access to information regarding engagements you are not staffed on, so you should not speak about your client even with your colleagues. Let alone talking with friends outside of the company: you can be sure that information will end up to some competitor who can exploit it in the sales process. Or to a friend of a friend's wife who is a corporate lawyer. It's not uncommon to hear of people given the sack, ,or big clients lost, for things like those.
Maybe second only to expense account cheating in the arena of motives for abrupt firings in my line of work.