Well it really does not matter it is 2014 or 1814, a legally binding contract is still a legally binding contract.
It's not as straightforward as that: not all terms in an employment contract are enforceable, as there is legislation that may take precedence.
People: I think some of you are being a little harsh on the OP, who came looking for some information and advice. There's a certain strain that is sometimes evident on this site by some commentators to the affect of "you signed it: tough on you, don't expect any sympathy, much less help".
As I said, all the OP was looking for was to know was the term enforceable, and advice on what to do.
As for the original query, some years ago I worked in a company that had a three month notice period. The reason it was there was that it was a small company that could be badly impacted if they couldn't plan the transition of people from projects with client companies. When I did leave, it was towards the end of a project, and I agreed amicably to leave when it finished (which I think was three or four weeks from the time of notice).
So, to those who say it's completely unreasonable to have such a long notice period, I'd say that there can be very good reasons for it, especially in small companies. To those who say you just have to stick with it, I'd say there's no harm in asking for a shorter period, and trying to negotiate a shorter term, which could well be acceptable or even advantageous to the employer. To the OP, I think it's possibly too late for such an amicable agreement, and you probably just have to stick with it, though I would make the effort in trying to reach agreement (for example, ask why it is needed, and see is there any scope for meeting their needs that doesn't involve the full period).