I always attach the sleeve after the binding is done.
Yep, I have to hand sew the top and bottom, but I find it lays much better, and the top doesn't fold over.
http://quilting.about.com/od/decorat...ilt_sleeve.htm
Regardless of which of these you choose, make sure you have a little excess fabric in the sleeve to take into account the space the rod or dowel will take up.
It will hang much straighter and flatter this way.
In the first method upthread (sleeve and binding together), don't make the excess more than 1/4" because it might show over the binding when hung.
Here are instructions for that method
http://quilting.about.com/od/decorat...t_sleeve_2.htm
If it's a really small wallhanging, you can probably get away with the triangles in the corner method.
http://seaside-stitches.blogspot.com...spiration.html
You can do this after the binding is attached to the front - before it's sewn down on the back.