As someone who sold their iPad 2 to put the money toward an 11in, perhaps I could give some input.
The fact is (in my mind at least), the iPad is all about fun. It's about content consumption. It's about doing the basic computing tasks in an ultra-mobile fashion, e.g. web browsing, Facebooking, Tweeting, checking email, etc.
That was my mistake. I tried to make it into a work machine. I kept all my course pdfs/lectures on it. I used a stylus to take notes on it. It worked for a little while, but it was forced. So after 4 months, I sold it and eventually got an 11in MBA. I need multitasking... in the sense that I needed to be able to have information side-by-side, be able to switch between windows constantly and quickly. The MBA was a perfect solution.
The 11in is, of course, a full computational device - you can use it to really do anything to your hearts content. It can do almost anything an iPad do, but there are three distinct disadvantages I've come across: No touchscreen obviously, worse battery life, and reading pdfs/books/lecture notes/etc. To be honest though, the advantages greatly outweigh the disadvantages.
In saying that, however, I think this year I might go with a 13in MBA and an iPad 3 just for fun. That way I can have an iMac at home for serious computing needs, gaming, homework, etc.. then an MBA for productivity on the go.. then an iPad for fun.
It's a tough call though when you have an iPhone. Sometimes laying in bed with my phone is easier than maneuvering around with an iPad. So meh, who knows.
Anywho, the 11in is an excellent machine. As is, imaginably, the 13in. Can't wait to see what 2012 brings.