I've just created a wifi network using an 802.11g Airport Express.
My 2010 Mac mini (running Windows 7 via Boot Camp), and my iPhone 4 (running iOS 5) both see and connect to the Airport Express's network just fine.
But my 2009 MacBook Pro (running OSX Lion 10.7.2) doesn't seem able to even see the network.
It has seen the network a couple of times since I set it up (about an hour ago), but when I tried to connect to the network, it rejected the (WPA2 Personal) password (which I checked was correct via "Show password") with a "Connection timeout" error after only about 3 seconds.
I've tried to join the network manually using the "Join Other Network" option, but it can't find the network then either. I've turned wifi off and on on the MacBook Pro, and restarted both the MacBook Pro and the Airport Express.
I must be doing something wrong, but I can't figure what it is.