I believe I have the answer to the problem. I have spent the last 8 days and nights trying to figure out why our iPhones and iTouch won't connect to our WiFi once we installed our Airport Extreme. Prior to that, we connected without a hitch. The story is much longer and more detailed than this, but I'll try to give you the Reader's Digest version.
We did a standard setup of Airport Extreme and 2 MacBook Pros, an iMac, and an iPad connected flawlessly, but the iPhones would not connect to WiFi. We kept getting the "unable to join network" message on our phones. Prior to setting up the Airport Extreme, we didn't have a problem at all with WiFi.
Here's the fix:
1) Go to Finder and click on Applications
2) Under Applications click on Airport Utility
3) Double click on the Airport Extreme that is being picked up
4) A screen will come up with horizontal tabs indicating Summary, Base Station, Wireless, etc.
5) Click on Wireless
6) Where it says Radio Mode--DO NOT choose automatic
7) Choose 802.11a - 802.11b/g
8) DO NOT choose 801.11 n--apparently the iPhones, iTouch, etc. can't pick up or read "n"; I'm not a technical person so don't ask me to explain. They pick up "a, b and g" but no "n."
9) Click Update
10) Go to your iPhone, etc., select the network, the key in the password used to protect it and you should be live and good to go.
I have a friend who is a computer forensics guy with the government who is THE Mac Daddy; he gets all the credit for this fix. We've been operating flawlessly since.
I hope this helps the many people out there who have been as frustrated as I have been trying to get all of these things to talk to one another. The Apple Support team nor Apple Care techs were able to help me.
Now I can spend my nights more productively not obsessing over this connectivity issue. Technology is great . . . when it works.
I still do love Apple! Once you go Mac, you never go back!