Even though Apple hasn't yet even announced the 2nd generation iPad, we are already hearing the first rumors of the 3rd generation iPad. Those rumors seem to suggest that Apple is planning on accelerating the iPad's release cycle with the 3rd generation model arriving as early as this fall (September).
The buzz started when Daring Fireball's John Gruber mentioned that the new HP tablet may run up against the iPad 3's launch:
If my theory is right, they're not only going to be months behind the iPad 2, but if they slip until late summer, they might bump up against the release of the iPad 3.
Gruber has been known to casually mention insider knowledge which he admits. In this case, however, he follows up to say that this iPad 3 comment was simply a guess, and not based on any actual knowledge. He gives his reasoning for such a quick turnaround between revisions:
Thus, my gut feeling is that Apple will move the iPad to a September release schedule, alongside the iPods. But they wouldn't want to wait over a year and a half from the announcement of the original iPad to announce the second one -- not with these stakes, and not with so many serious competitors trying their best to catch up.
While that may have been the end of it, TechCrunch claims that they have independently heard that Apple will indeed be revising the iPad this fall:
We've now heard that this "fall surprise" is related to this would-be iPad 3. We don't have any more concrete information beyond that. But, as of right now, the plan is apparently to release one iteration of the iPad in the next few weeks. And then blow the doors open with another new version in the fall.
Apple typically refreshes its product lines in predictable patterns. The new iPhone tends to be announced around WWDC of each year, while the iPod line is refreshed around September. This has been the first year of the iPad, so everyone is expecting them to follow a yearly pattern around April. It seems Apple is following that timeline for the first revision, but may end up changing those rules this fall.