Gruber tosses his guesses into the ring:
iPhone OS 4.0 will only be for the iPhone and iPod Touch, not the iPad. The iPad, like any such project at Apple was developed by a team that was locked away in secrecy, so the team working on iPhone OS 4.0 only found out about the iPad when the rest of us did, on January 27; Dalrymple has it exactly right: OS 4.1 will be the unified OS for all these devices. (My spidey-sense tells me that iPhone OS 4 is going to drop support for first-generation iPhones and iPod Touches, and some of the features may only be available on the 3GS and this year’s new models.)
I’m pretty much in agreement. I am curious to see what the “unified OS” would look like. The UI advances Apple made with the iPad wouldn’t scale down very well, I don’t think. I think the time has come for first-gen iPhone owners to upgrade.