There are so many little ballerinas out there, and some not so little! I have tried to come up with some ideas to offer you more than just ballerina wallpaper and pink everything!
Use traditional ballerina colors in your fabrics, and a very understated color on the walls. This way the little girl won't outgrow the room quite as fast, and it will be easier to change over to a new theme without having to repaint! Try using sheers, and tule types of fabrics at the windows for texture and "feel". They also happen to be very cheap!
Buy cheap closet door type mirrors at youe home improvement center and install several of them on the wall, studio style. If you choose, you could even use closet doweling and some brackets to set up a "barre". Just be sure to secure the brackets well in the wall studs!
There are many greeting cards etc... now that feature toe shoes, ballerinas, etc... and are quite nice photographs. Frame them up as artwork for the room, or frame some perfomance music, perhaps from their last recital?
Instead of cutesy wallpaper border, find a toe shoe stamp and design your own wall design. There are stencils too, but be careful with older kids... the ribbony, frilly type usually available can appear too "babyish" to them. Also consider stamping a key piece of furniture, or even the window treatments.
Try expanding from the usual pink into blues and purples also, to give the room a more unique appearance.
Remember, you can make her feel like a real "grown up" prima ballerina in her own room!