So how would this be implemented? I'm thinking of having ads displayed in a panel similar to the chat, crew and other panels. If you've noticed, when you start up zOMG!, the crew panel is by default closed. That space could be used to display the ad panel. Of course, the ad panel itself should have a button and possible shortkey to open and close it.

Merits:
An easy way to generate in-game revenue
Not be terribly annoying by being in an easily removable panel
Easier to implement than in the game itself and much more viewable
Demerits:
Finding in-game advertisers.
zOMG! might not have the amount of traffic to generate enough clicks on ads.