The latest iOS 6.0 has just been released 5 minutes ago.
You can update it from the settings within iPhone/iPad or through iTunes.
According to Apple, it has up to 200 new features for the older phones. It includes Apple designed maps, Siri updates (both are for 4S). There will be Facebook integration, Passbook, Shared Photos depending on your iPhone generation.
We are updating our old iPhone 3GS to iOS 6.0 right now. The phone runs on M1 Singapore network and it will take a while to complete the download.