I've seen tons of cheaters in CoD4. Dunno what servers you play on
EvenBalance refused to admit that PunkBuster didn't work on Windows 7 for months of people reporting issues because it was "pre-release" software. Even now, you can't just install it. You have to find some hidden installer that actually installs the services properly.
EvenBalance also refuses to admit that their third party software scanner doesn't work properly and is often too aggressive in recognizing potential hacks. The Steam beta UI triggers hack detection in PunkBuster, and their response is that it is Valve's problem and they shouldn't have to do anything with it.
Aside from that, PunkBuster is one of the weakest anti-cheat applications out there. If you're not seeing many cheaters, it's because they've moved on to other, more popular games to hack in.
Brizz is mad @ PB. While I'm not the world's foremost authority on the subject, PB works just fine on my Win7 rig. I didn't have to screw with it at all. Also, with the game (BC2) being new, it actually hasn't been banning people right away. It sure appears that they've been redflagging things, and then once theyve confirmed its a cheat, then the bans start flowing like water... Again, just my recent experience...