I tried to make the switch from dual to single monitors earlier this year. It's really nice to only need a laptop (or one monitor + kb/mouse) on your desk. I also find the border between the two monitors really interferes with my ability to use the space, somehow. During the experiment, the only time I ever found myself wanting a second monitor was when I was testing in the browser, and needed to view a whole web page, plus an inspector. Since I have periods where I do that a lot, I now have a second monitor hooked up to my laptop. The second monitor goes unused abou 90% of the time.

I think my ideal setup would be to have a very high-resolution display like an Apple Cinema display.