Ok so I have been battling with the same issue for a while now and have finally managed to crack it.
I have a Huawei 3G router that has been working for my PC;s and iPhone for a few years now. I then bought an iPad3 (3G and Wifi) and could just not connect it to the wifi network.
Fortunately I have a spare Linksys router (WRT320N) which has a setting to change the Wireless band from 2.4GHz to 5Ghz with 20/40MHz pipe. After changing this no problem iPad could connect.
So now I have 2 wireless networks running with internet connectivity only on the 3G so I plugged a lan cable from the 3G router into the internet port on the Linksys, set the Linksys up with a static internet connection and now I can access the internet via this method on the iPad.
Funny thing is, my iPhone cannot connect to the Linksys my laptop can however. Anyway so now I connect the iPad via the Linksys router and the iPhone via the 3G router.
I think Apple have pretty much messed up with the wireless bands on their products.......it would appear that they don't understand ? If they did they would make it pretty clear and from my efforts to try and sort this out i see there are many people out there, like me, who are laymen and there is no where that this is explained properly.
Most of the installed base of wireless routers I would think are on the 2.5GHz band, I say this beacause there are virtually NO wifi networks out there that I can connect to.
This is a huge frustration and a pretty poor reflection on Apple. So much for MOBILITY!!!!!