I am sure the tablet devices will follow a similar cycle as iPhone. Upgrade once in 2 years. I am beginning to wonder, the only aspects that might make me want to upgrade to IPad 4 next year, could be faster processor and more memory, thus making the overall experience better. I am sure Apple will have some exclusive software that will run only on their latest device (ex: Siri), that might force me to upgrade. Given how I use the Ipad right now - Videos, Netflix and eBooks (very limited browsing), I am ok with what I have now.
Will be good to know some statistics on how many upgraded to Ipad 2 from Ipad 1.