Just a note you can search for more threads about it on this board and see lots of great pics and info. It is called the Bibbidi Bobbidi Boutique but the main thread is spelled Bibbidy Bobbidi Boutique (I think).
Make a reservation if possible. They do allow walk in appointments but who knows if they will have any openings.
The info is:
For reservations, please call (714) 781-STYLE or (714) 781-7895.
We had a later afternoon appt because we were getting dresses and they can be a little uncomfortable to be in all day. If you aren't dressing up then perhaps a morning appt would be better so she can have her hair & makeup all day and get LOTS of comments. Or if you & she don't mind being in the dress all day (we worried about it getting torn/damaged on rides). The CMs in the parks will call her princess throughout the day. Also, they do scrape the hair back in a very tight ponytail to make many of the styles (espec the bun princess style). Make sure your 3 year old doesn't have a sensitive head or she may hate the whole experience. We went to Ariel's Grotto for dinner afterwards and it was a great "princess" day.