While Apple will be serving up a hearty meal with iPhone OS 4.0, not all iPhones and iPod touches will be able to offer up the full experience. Only the iPhone 3GS and third generation iPod touch will be capable of handling all seven features highlighted at the event today (multitasking, folders, mail enhancements, iBooks, better enterprise features, Game Center, and iAd).
While the iPhone 3G and second generation iPod touch "will run many [of these] things...there are some things they won't run," according to Steve Jobs at the event. The most notable of these things is multitasking, which Apple said the iPhone 3G and second generation iPod touch just can't handle (of course, we know they can on jailbroken units, but Apple didn't acknowledge that possibility). There was no mention of the first generation iPhone and iPod touch, with the implication being that if you want multitasking on the iPhone, it's 3GS only... or perhaps some new hardware that might be announced later this year.
As one who balked at the iPhone 3GS -- both because I didn't qualify for AT&T's upgrade price and because I don't find its features compelling enough to justify a purchase -- this certainly shortens my purchase cycle. There are probably many in the same boat. While I was going to purchase the next generation iPhone anyway, I'm now contemplating upgrading to an iPhone 3GS or better.