Valve is debating on weitehr or not to have ads in CS 1.6. If you've played games like PlanetSide or Splinter Cell: Chaos Theory and Double Agent you'll notice that there are dynamic adds that change daily, weekly, or monthly.
If 1.6 is allowed to ahve ads it's only a short amoount of time before CSS gets them as well. I will refuse to play CSS if it has ads in it.
Games like Chaos Theory use spyware to see what type of websites you visit recently and make posters, tvs, and banners in-game match what you have looked up. This is rediclous (sp) and I can't stand it. Read more here... I dunno how to use HTML sry.
I actually really like the idea of ads in them cuz then it adds to realism as long as the ads are placed realistically. They are a part of everyday life and you see them everywhere you go and when you play games there are none and it makes the game even more fake.