I have the same problem. When my MBP wakes up after sleepmode, Safari says I'm not connected to the internet while I am connected to the internet. And the Wi-Fi is working fine on my iPad and iPhone.
This happened after I updated to Lion 10.7.3. I allready tried the 10.7.3 combo update, but the problem didn't go away. And I tried re-installing the original Lion from the recovery disk, but then it installs 10.7.3 again.
Does anyone know a solution?