Apple gives 2 iOS updates per device.
iPhone 3GS - came with iOS 3, gets iOS 4 and 5. No more.
iPhone 4 - came with iOS 4, will get iOS 5 and 6. No more.
iPad - came with iOS 3, got iOS 4 and will get iOS 5. No more.
iPad 2 - came with iOS 4, will get iOS 5 and 6. No more.
Thats the way it works. They can't give you three iOS updates because then people wouldnt buy new iPhones and iPads. Also, RAM is a huge factor. iPad 1 with 256MB RAM won't get iOS 6 because surely iOS 6 will bring new features that will need more RAM.