Pretty much as the title says. I'll likely use Monotouch for some/all projects if that makes a difference (noting this still needs a Mac).
I've read this Question...
Macbook Air for iPhone Development?
...and see the related answers note performance issues and buggy/non performant screen spanning issues on previous generation models.
I'd like to know if these two issues are resolved with the new specs.
Observations on performance running Win7 and WP7 under bootcamp are welcome but not required. Related question raised here looking for specifics on WDDM1.1 compliance on this model if you care to comment there.
edit: formating/highlighting.
As both my answer and Marco Arment's in the other question indicate, the previous generation MacBook Air was more than capable of running all the developer tools. In fact, Marco's review was based on the original MacBook Air, which was even less capable than the second-generation one that I had.
Performance is a relative metric, and it only improves with these later models. The second-generation MacBook Air that I had was never the bottleneck in my development process.
Pretty much all of the bugs were worked out by the second-generation model that I had. I noticed no issues with driving external monitors from the system at all. Again, Marco was basing his review on the very first build of the Air, which had some known issues.
Screen size will be an issue on the go when developing with many windows open, or trying to simulate the iPad, but that would be the same with a 13" laptop of another type. I imagine you'll be connecting this into an external display for extended development at your desk anyway.
The only reason I no longer am using it as my primary machine is that I found I didn't need the portability after getting an iPad.