I did not set out with a theme. I could not bring my self to do a cutsie baby room, it is just not DH and I. So I went for colors and funky fabrics. It is turning out to have a bit of a bird and tree theme.
One tip on doing a mural. Find a photo or outline you like on line. Print it off. Trace over it in one inch squares. Then you can take each square and translate it to the wall in one foot squares. It helps keep it a bit neater. Or you could see if you can rent an overhead projector.
http://www.ehow.com/how_17473_paint-wall-mural.html