As I mentioned in the part 1 post, I managed to get things working, but the Airport Express (AEX) was still connected to the Ethernet. As soon as the ethernet connection is gone, the Airport Express and my airport on my MacPro are no longer getting a DHCP lease. Giving the AEX a fixed IP does not solve this (though the amber blinking light turns green).
So the people who told us that it would not work are right!
Update 24-05-2010: It only works if the Airport Express is connected to the LAN through ethernet. Wifi only does not work! Will look into this!
Today I managed to extend my Thomson Speedtouch Wifi network (by UPC) with one of my Airport Express routers. I have tried this once before and failed. Somehow the wizard in the Airport configuration utility is not very helpful. But this time I was more persistent, and got it to work!
If you want this working also, you should try the following:
