I'm gonna take the side of neutrality here - it's all about perspective. It depends on what you think sexy is. If we're talking sexy in a physical sense, I'm sorry to say girls, but this is an outright lie. While some smart girls may be physically sexy, attractive, whatever the bottom line here is that this is a huge generalization. Not all smart girls are going to be sexy. Some dumb girls are going to be sexy as well. I feel like I'm pointing out the obvious here, but no one has done so before me on this node. Intelligence and sexy - at least in the physical sense of the word - don't really have anything to do with each other.
But then there's the fact that smart can mean sexy. As for what Jaez said above, I totally agree with you there. There's nothing that turns me on more than when a girl speaks her mind and she leaves you completely baffled with her wits. It is sexy in a way - a way that no dumb girls could hope to achieve. Engaging in a philosophical discussion with a smart girl is sexy in its own way - but making love to an extremely attractive dumb girl is sexy in a different way. It's quite incomparable, although I personally would be inclined to prefer the former.
There are two more points I would like to bring up. First of all, not all guys are going to agree that smart girls are sexy at all. There are certainly some guys who really don't like smart girls and for whatever the reason maybe (perhaps the girl is smarter than they are), it turns them off. To them, smart girls are very far from sexy. Secondly, there's the smart bitch factor. There are literally a ton of girls out there who are extremely intelligent - but total bitches. Let me be the first to point out the snobby rich type. While they may be smart, they can be way too full of themselves, and this can be a total turn off. It all depends on how you look at it.
So, in conclusion a lot smart girls are sexy - there's no question. But not all people think that, and it all depends on how the individual defines sexy. And of course, there are the smart idiots to consider. But, in a way, there's nothing more erotic than having a long talk or even being surprised speechless by and incredibly smart girl with an incredibly intelligent thing to say - except maybe if she's smart and got the looks to go with it!