>I have an ethical dilemma.

You may not be contractually obligated to not be hired, but the client may be contractually obligated not to hire you or to pay for it. And if you're client doesn't know this or forgot this, I bet they'll be mad that you, as a representative of the consulting firm, didn't know or tell them of this.

Now, if they're not contractually obligated to pay a finders fee or something, I know lots of consultants who did this very thing. Sounds like a legal, not an ethical question. The ethical obligation -- and smart thing to do -- is talking to the client about any potential legal or financial harm they may suffer by hiring you.