During the Google Developer Day 2007 in San Jose (and around the world) a very exciting and profitable announcement was made regarding the optional addition of location based advertising in the map viewport. These ads will appear as pseudo (my word) markers and when clicked will bring up an add that if clicked on, will generate revenue for the AdSense partner.
You can get a taste of how location based embedded ads will work by going to maps.google.com, try this url:
Check out the BART stations, you'll see that when you hover over them your mouse changes to indicate a hot spot, clicking on it brings up an infowindow just like a marker. In fact, it isn't a marker but a "pseudo" marker that causes a round trip request to the Google server to get the meta data for the infowindow. Very clever, very efficient,
The rollout of this feature will be sometime in the month of June.