Just finished my attic studio (still not painted and I've got about 80 acoustic tiles to stick to the pitched ceilings) - floors are decoupled from the house - hanging from joist hangers above the pre-existing joists from RSJ steel beams. The room is prettty damn soundproof (no sound to downstairs when recording or mixing at pretty loud volumes), important given the wife and kids are usually sleeping below while I shred above .
Desk is home made along with the built in rack cabinets, got plans from the net and altered them to suit (its about 8ft wide which is pretty big).
Also threw in a few pics of my axes if you want to hit the arrow buttons to flick forward. Will post some pics of the completed room in a few weeks when I get a chance to paint it out etc.
PS that it with the lights on, I usually record and mix in the dark with screen light only - much better ambiance!
If anyone is interested in converting their attic space and wants some tips just let me know - I picked up a lot of tips and learnt through trial and error - could save you a lot of time and money!
Here it is on Google's Picasa: