Could you direct me to some of this research you're citing? it seems really unlikely to me that dual monitors would have any kind of significant impact on productivity. in fact don't they encourage multi-tasking which tends to decrease productivity?
How does it play on people who have ADHD?
I've been on a single 13" for three years and I don't want to go back to two 21" again.
Might go for an 15" just because of xCode, but that's it.