At the e-consultancy paid search round table all the agencies were getting excited about the ability to use demographic targeting in MSN Adcenter, so obviously Google have realised this is a hot topic and in the United States at least integrated demographic data from comScore Media Metrix about individual websites into the site targeted options.

Google says "If your product appeals to young women, for instance, you might want to target sites popular with the female demographic, the 18-24 age demographic, or both."

https://adwords.google.com/support/bin/answer.py?answer=33743&hl=en_US