As I'm exploring advertising solutions, I've discovered that most ad networks aren't really appropriate for a site that hides most content behind a registration

As I'm exploring advertising solutions, I've discovered that most ad networks aren't really appropriate for a site that hides most content behind a registration or is highly personalized, like a social web app. This is because most ad networks (AdSense, YPN, AdBrite) rely on spidering your content to do their targeting... and they don't log in to your site to do it. If they can't get into your site to see the content, they can only show run-of-network ads, or ads based on what you tell them explicitly about your users in general. Is there a product out there that is more suited towards showing targeted ads based on protected page content? Or passing keywords as parameters to the ad script? There are definitely privacy issues here, but I thought I'd ask here to see if anyone knew of a way to show my users relevant ads, and not just "hey I think you're a 20-something with lots of disposable income" ads... Maybe I have to do another startup just to serve this market?