Having this problem very often lately. Internet will work just fine, then all of the sudden i can't open a web page. I have found that toggling airport off/on will allow it to start working again, but that gets pretty annoying.
Ran into this issue a lot on Uverse and started using OpenDNS which fixed it.
Yes, me, I'm on the 10.7GM, thought it might be a bug in that. I get is most when opening the laptop and resuming from sleep.
I also rebooted my time capsule to see if that makes a difference.