The official statement says iOS6 will be available sometime in fall.We expect it to arrive in September.
What devices will be compatible ?
iOS6 will be compatible with iPhone 3GS,iPhone 4/4S,iPod Touch (fourth-gen ),iPad 2 & the new iPad.
How to upgrade to iOS6?
When you update,all data is first backed up by iTunes.After the upgrade,data is restored automatically.
Facetime will work over 3G (it was earlier Wi-Fi only).The App Store gets a new interface and your phone number and Apple ID will be unified you can get notifications/take calls on any authorised Apple device.
iOS6 will be available as a free update for all compatible devices.