Generally speaking Apple keeps a product in iOS update rotation for 3 cycles. EX iPhone 3G iOS 2.0-iOS 4.2.1. However this trait is going to change with the release of iOS 6. The iPhone 3GS will support with limited functionality iOS 6. So going off this, your iPad will most likely support up until iOS 7. iOS 4-iOS 7.
As product become for tech spec impressive, you will see them being supported for a longer time. iPad 2 to will most likely support iOS 8 possibly iOS 9 (wow thats a far time into the future!)
Correction: Your math is wrong, by your calculations using iPhone 3G, it would be iOS 4 - iOS 6 for iPad 1, but that is not even right because the iPad 1 originally shipped with iOS 3. So by your rationalization it would be iOS 3 - iOS 5.
On another note, my iPad 1 runs horribly on iOS 5. I doubt it will get iOS 6. As soon as iPad 3 is out, i'm upgrading. I can't stand the 1-2 minute delay to open the Music app on iPad 1, and the delay in registering touches.