I've now eaten at Ming Tree on Washtenaw Ave. a few times. Compares well with Siam Square. If you want spicy, try the Ga Pao Guy (every Thai restaurant has one or two dishes that you order if you really want hot food; this is apparently theirs). One surprising and disappointing hole in their menu: no Tod Mun. Note that Ming Tree also serves standard Chinese food--you have to ask for the Thai menu.
Tuptim (also on Washtenaw) is now open, but I haven't tried it yet.
Can't believe I forgot to mention Thai Bistro (on Ford Rd. in Canton) in my earlier rundown of good Ann Arbor-area Thai restaurants.