I have been doing so for about 6 years. In my experience, gaining screen real estate is amazing at first, and losing it really hurts at first, but in the long run, it doesn't make much difference.

My dream machine was a 12" Toshiba Portege with an extra battery, 1920x1200 rez and a trackpoint.

Great form-factor, especially on planes.

It eventually started falling apart, and I never found quite the same thing again.

My 13" Macbook Air serves me well as my primary development and design workstation. For purely development, I would love to use an 11" Air, but Photoshop is a bit too cramped on it.

My 27" display gets used for watching movies mostly these days. I do go through phases though, and will probably try using the large screen for work again eventually. For now, I find the smaller screen less distracting.

I do it on 12". Absolutely no need for any more for development, not even one bit.

Now, design work obviously needs a lot more, but that's offset by the fact that my HP 2730p is also a tablet with stylus, which made me extremely productive in dishing out graphics.

I develop on an 11" screen.

Yep. I've had 13in and 15in MBPs but I prefer my 13in MBA. Work gives me the option to get a 24 in monitor for my desk but I haven't bothered to request one.

I develop on a 12". Well, 12" multiplied by 20 workspaces.