Same problem with my iMac 27" i7 and airport extreme.
After waking up from sleep, no internet connection, while all other devices on the network (macbook, windows laptop, iphones, PS3) are fine. The iMac has network access - I can ping other devices, just no internet access.
What seems to be causing this is the "wake for network access" setting in energy saver. When turned on, no internet after wake up. When off, internet connection's fine after waking up I can consistently reproduce the problem. When the setting is off, of course, one cannot share printers, music libraries, etc., without first waking up the computer manually, which is annoying.
I'm pretty sure "wake for network access" was not causing this before. I've been using the iPhone remote app to control iTunes (although I haven't used the app lately) and the iMac would wake up and play online radio stations without a hitch. I suspect Apple introduced this bug recently.