I allow Google to place small, relavent advertisements on my IRLP HelpSite.  My privacy policy allows for the policies outlined in Google's own advertising privacy policies.  Specifically:

• Google, as a third party vendor, uses cookies to serve ads on my site.
• Google's use of the DART cookie enables it to serve ads to my users based on their visit to my sites and other sites on the Internet.
• Users may opt out of the use of the DART cookie by visiting the Google ad and content network privacy policy.

Please see  The Network Advertising Initiative, or NAI, which explains the guidelines for responsible data collection of non-personally identifying information.  Specifically:

"We use third-party advertising companies to serve ads when you visit our website. These companies may use information (not including your name, address, email address, or telephone number) about your visits to this and other websites in order to provide advertisements about goods and services of interest to you. If you would like more information about this practice and to know your choices about not having this information used by these companies, visit this link."

You can find additional information in Appendix A of the NAI Self-Regulatory principles for publishers (PDF). Please note that the NAI may change this sample language at any time.