The new MBA 13 screen is 1440x900, a nice step up from the 1280x800 that 13-inch MacBook users have been used to.

For web programming it is fine. Reddit was programmed on a 13" MacBook, after all.

If you're doing Xcode development I might still be hesitant. The rapidly upcoming Xcode 4 environment is one large window and seems to put smaller screens at a significant disadvantage. It's probably okay, but I would really want to see it running.

Yes it is. I am using my x301 as primary development machine. The x301 as well as the MB Air have 1440x900 which is enough for me.

It's not just about resolution as some have pointed out. I like to have Terminal, emacs, XCode, Firefox, Preview for any PDFs all open at once.

That's why I prefer 15/17" + external 22/24" - I would never be able to handle the small 13" just by itself.

I generally am using 8 to 10 Spaces desktops at a time, with several of those desktops running multiple windows. With Spaces and Expose I find it good enough on a 13" screen.

Bigger is always nicer, but the tradeoff is that it would be a lot harder to carry my laptop with me. I prioritize mobility.

I only use 2-3 Spaces (to separate work from chats and distractions). I find it hard to split up "work" into multiple Spaces when I need to look at all of my work at once (ex. need reference docs + code up at the same time).

FWIW, I write code on my 13" MacBook all the time.

I guess it depends. I worked on a 13 inch MB screen for three years before getting a 15 inch MBP. I am much happier with the 15 inch. This is mostly because I use vim with 4 to 6 split screens at the same time.