3 displays, from left to right:
19" 4:3 Dell LCD at 1024x768 for the Reason Rack, Connected via a USB Display Adapter
30" Dell at 2560x1600 for the SSL, connected via Thunderbolt with an Apple Displayport adapter
27" iMac at 2560x1440 for the Reason Sequencer
All in all it works quite nicely. My only real complaint is the lack of resolution-independence in Reason itself. I have to have the leftmost monitor at 1024x768 to be able to easily read the devices in the rack, and still sometimes that's trick because the screen is physically about 3 feet away from me. And on the higher-resolution displays, everything is way too small and almost unreadable unless I lean way in. I can't wait until Reason goes resolution-independent so we can adjust font sizes etc. in any way we need.
(Oh and in this photo, I have the SSL on the right and sequencer in the middle... I switched that around right after I took it.)