I'm probably not going to be the first one to mention this, but I want to go on record as early as possible that the single "killer app" for the newly released iPad is college text books.
I've been an iPhone 3GS owner since the day it was released and have loved every second of it. That is in stark contrast to the prior years of owning Palm and Windows Mobile phones, of which to say were terrible is in understatement. I've also been a huge iPod supporter for years now too, having recently bought my third one.
That said, I personally don't see any use for the new iPad. I currently use my iPhone for email and web surfing, and could see where using an iPad would be nicer, given its bigger screen. But I would never take the iPad anywhere other than the living room couch with me, as the form factor is simply too big (compared to the iPhone which nicely fits in my pants or jacket pocket). And since I can type on a regular keyboard at over 60 words per minute, I don't think that the iPad and its 10" screen is going to replace my regular desktop/laptop with its 24" screen anytime soon either.
But back to the college text book thing... if Apple were to be able to pull off selling college text books (and K-12 text books too) for use on this thing at say 25-50% the normal price of hard copy text books, the iPad would pay for itself in one or two semesters. And students get to stop carrying around the back breaking backpacks filled with the text books that they currently have too. And that's not even taking into account the added technological aspects of having ebooks that you can mark up as you please. Now THAT would lead to Apple selling millions upon millions of these things. Maybe that current stock price of $208 is low after all...