I offer a prediction: the next version of the iPad (as I'm calling it, the "iPad 2") will ship sometime in April 2011, probably more towards the end of the month. I don't have any insider information, this is just an educated guess.
Now, on to the why: In late January of this year, Apple announced the iPad. But it didn't actually become available to buy until April 3rd of this year. The 3G version came along even later, on April 30th. The reason why Apple let so much time elapse between when they announced the iPad and when they actually shipped was due to the need to give developers time to have some iPad-optimized applications ready by the time the iPad reached customer hands.
The next time around, however, there will already be at least 25,000 iPad apps already on the market (a figure that Steve Jobs announced in September), so no additional lead time will be necessary. Thus, my guess is that the iPad will be more like the iPhone.
The iPhone was originally announced in January of 2007, but didn't ship until June of that year. Since then, every June or July, a new iPhone has been announced, and it has generally shipped a few weeks after the announcement. The iPods have fallen into the same pattern, typically being announced (and shipping) in September of every year. Both of these products are refreshed on an annual schedule, and I see no reason why the iPad wouldn't fit into the same pattern. And it would even have a nice symmetry: Apple would have a major product refresh in the spring, summer, and fall, every year.
And if I'm so right about the timing of the releases of Apple's iDevices, how come new Mac releases have such an unpredictable schedule? Well, I have a theory about that too: for the Mac, Apple is dependent on external suppliers for key components (Intel for CPUs, NVIDIA and AMD for graphics chips), and as such, Apple is at the mercy of these vendor's timelines. I bet that this is yet another reason why Apple is taking things more in-house with the A4: they'll be much more able to control the timeline for all of the components in their iOS-based products.
-Andy.