I have a similar, but not exactly identical problem.
My set up:
Wired Buffalo Hub connected to Optical Fibre internet.
MacPro plugged into one connection off the hub.
Epson wireless print server plugged into one connection off the hub (it won't connect wirelessly so is used configured for wired operation.)
Dell connected to the last connection off the hub.
Airport Express connected to another connection off the hub.
Connected to the Airport Express are:
Wired - Airtunes Speakers, USB Printer
Wireless - G4 Powerbook running 10.4, MacMini Running 10.5
In addition, the MacPro also connects to the Airport for inter-machine file-sharing and iTunes.
My problem lies with my MacBook Air which was perfectly happy to connect to the internet through the Airport until this week when it suddenly stopped, even though it is connected to my Network (Password protected, WPA2 encryption.) I have not changed anything, added anything nor updated anything between everything working swimmingly and everything not. All software is at the latest version, as is firmware.
The status is now:
All machines can see the network.
All machines can see the printer and Airtunes
All machines EXCEPT the Air have 10.x.x.x IPs from the Express by DHCP with NAT. The Air has a self assigned IP.
All machines except the Air can connect to the Internet wirelessly, including the MacPro when I tell it to use the Airport link as its connection.
It seems that the Air is either not requesting an IP address, or not recognizing the answer when it gets one, or the Airport Express is ignoring it. The Air connects perfectly well through the USB LAN adapter or off every other WiFi spot I have tried it on, all with the same Network, Firewall and Little Snitch settings. I have rebooted, reset and re-anything else I can think of the Express and the Air. If I turn the Firewall and Little Snitch off on the Air I can, maybe, get a connection, but not every time.
I am assuming there is a corrupt preference file somewhere on the Air, but I don't have the first idea which one it would be.
Any suggestions gratefully received!