iPod touch 1st gen and iPhone 2G got 3 years support. (1.0 to 3.13)
iPod touch 2nd gen and iPhone 3G got 2.4 years support. (2.0 to 4.2)
iPod touch 3rd gen and iPhone 3Gwill get (at least) 3 years support. (3.0 to 5.x)
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.
The caveat for the iPad is that it was released very late in the iOS 3.x cycle. Literally just a couple of months before the iOS 4.x launch. As a result it's hardware closely matches the 4th gen iPhone/iPod Touch with it's A4 CPU/GPU. That said, I would be a bit surprised if the iPad does not receive some limited iOS6 support.Apple gives 2 iOS updates per device...