Here's my story.
At the beginning of October, I had an iphone 4s. My wife has a iphone 4, We also have an ipad 2, iphone 3g, and original iphone. At that point, everything worked wonderfully on our wifi.
Then, I made the mistake of updating my iphone 4s to iOS6. Wifi immediately stopped working. I thought it was just a bug apple had when you had the 4s/ios6 combination and they'd fix it in ios6.1. I also figured that I'd be getting the iphone 5 in a few weeks and could live withit. Surely, they wouldn't release the iphone 5 having the same bug where it's unable to connect to wifi. Fast forward though a very annoying few weeks of no wifi, I get my iphone 5. Does it work on our wifi? Nope. Of course not. On either phone, nothing is grayed out. It just simply won't connect. I've tried all the methods listed in this discussion... toggle airplane mode, restore the phone, reset network settings, etc. Nothing fixes it. I've even tried changing the encryption on my router.
Since none of these ideas have resulted in anything workable, I did more research. I've found a few sites that list these two possible issues.
1) ios6 will only allow authenitcation with ipv6 routers. ipv6 is relatively new and so older routers are still at ipv4. This seems to be the problem with my phone because I can connect at a friends house to his newer router. However, it's rediculous that apple would force it's users to buy new routers simply because it fancies a newer protocol. If this was the issue, I thought surely they would fix this is ios6.1, but they didn't. I'd think this would have big ramifications for apple's users. Think about it. Now you can't use your wifi at home. You can spend $50 and buy a new router and your phone will work. But, you can't gauruntee that every starbucks, barnes and noble, airport, etc is going to upgrade their routers just for apple. So, you are forced to use your cell data plan. I'd think that could easily be construed as shady business practices (I have no idea what the legal jargon would be) in the court system.
2) some iphone 5's have apple wireless chipsets that don't work, but some have intel chipsets that do. Looking up the mac address (as the site advises) shows that I do have the apple chipset, but since I could connect at a friend's house, I don't think that's my issue.