I know that I never really followed through with the (former management's) plan to sell advertising through a central area on the site... In fact, this area has been listed under construction since I recoded the site back in 2002.
I'm still not sure if or when we'll sell advertising, but I did complete the majority of an advertising information portal on the main site; as this section will remain under construction (as we're not planning on selling ad's at this time (other than our current Google Ad's)) for those who're curious you can see the new system here
If you just try to access the Advertising System
all you'll get is an "Under Construction" placeholder; so you'll need to be sure to visit through the advertising preview link above.