As others have said, a lot of people already own iPads and there's nothing about the current concept/design that encourages especially upgrade cycles. Not that there's anything wrong with that from a consumer perspective. It does what it does pretty well and doesn't really call out for incremental improvements to battery life/resolution/processor power/etc. although those do naturally come about with each iteration.…
Vodafone in the UK will let you do this (with no extra fee). They've offered me it several times, but I didn't buy an iPad with a SIM slot.