After many months of expereince this TERRIBLE issue, reading these threads, doing some research and some experimentation, I offer the following two points for you:
1. As an electronic engineer, I was deeply deeply sceptical of any thoughts of freezing or heating my phone with a hair dryer. However, after trying unscuccessfully to downgrade to iOS5 on several occaisions, I did try the hairdryer treatment. It worked - for about 5 hours and then the problems came back. Out of total frustration, I left my phone on top of a radiator in our house over night. The heating comes on at 5am. At 7am I picked my phone up (it was pretty hot to touch, but no cosmetic damage). WiFI came on instantly and connected. That was 5 days ago - it's still working. Make of that what you will but if it helps you, I'm happy.
2. Before the success with the above radiator method, I was building up to go into the Apple Store in London to demand an out-of-warranty replacement. Did a little research on the subject and found out that last year Apple were forced into complying with the 2 year warranty for all consumer goods in the European Union. Take a look here : http://www.apple.com/uk/legal/statutory-warranty/ . This provides a 2 year cover for faults that were present when the product was bought by the customer. If you're in the EU this might work in your favour.
Good luck!
