I had the same problem. Cablevision came today and upgraded me from 100mbps service to 300mbps service. I got 300+ directly out of the cable modem, but as soon as I hooked up the Airport Extreme, I could get no more than 100mbps, ethernet or wifi.
Eventually figured it was a router issue, so I called Apple and after getting sent to Level 1 tech support, did hard resets, upgraded, then downgraded firmware, tried a dozen different things. Nothing. It was elevated to Engineering, and I was told to expect to hear from them next week.
After a bit of thought, I suspected my ethernet was being capped at 100base-t, so started considering how I could test that. Suddenly I remembered that back in the day, the switch always had to be connected to WAN/cable modem via a crossover cable. Hm.
Dug in my ancient wire nest in the basement and found a crossover cable. Connected the cable modem to the Airport with it. Bam. 300mbps at the desktop, ether and wifi. 300mbps to my PHONE.
I don't know what the issue that makes this necessary but a crossover cable is a quick (and for me permanent) fix.