Apple has supported iPhones and iPod touch for two years, so the iPhone 3g and the 2nd gen Touch did not get the latest update.
That's a really short product life-cycle, but it makes sense for phones because people commonly buy two-year contracts. But I really don't know what Apple is going to do. Although the A5 chip and GPU in the iPad 2 are significantly more powerful than the A4 in the iPad and iPhone 4, app developers aren't going to want to exclude the potential market represented by the fifteen million original iPads in consumers' hands. App developers will probably also want a more stable, standardized platform.
We've also seen the current generation of video game consoles outlasting previous generations, because developers are limited now by the resources and manpower necessary to produce high-resolution, high polygon art assets, rather than by the hardware. It's not at all clear that game developers will be able to take advantage of the graphics capabilities of subsequent iPad generations at app-store prices.
It's also not clear that gaming will drive most iPad users to upgrade their devices, so we may not see such dramatic processor and gpu scaling in the future. The iPhone 3GS and iPhone 4 have similar CPUs, for example.
It's also not clear that the iPhone 5 will use the same processor as the iPad 2. Tablets have much larger batteries than phones, and we don't know that Apple can hit it's battery-life target running the iPad 2 hardware on a phone-sized battery. If the internals of the tablets exceed the phones, then the tablets might get supported for longer. It's also possible that the current gen phones will be supported for longer than previous generations.
I also have the impression that people replace their computers less frequently than their phones (which are heavily subsidized by carriers). People tend to replace phones every two years when their contracts expire. I expect that many people will want three or four years of service out of a device that can cost over $800, so Apple may not be as quick to switch off the service.
On the other hand, these products have a definite life-cycle. The batteries in the iPad only last about 300 full charge-discharge cycles before they're spent, and it's not clear that Apple will be offering to replace those. On laptops, batteries are generally screwed in and removable, but the iPad's battery can only be accessed by removing the front plate, which is held on with glue. To get inside, you have to melt the adhesive. This thing isn't designed to be serviced, it's made to be replaced.
