Do you have a more precise schedule for when you plan to implement this (and other DataAnnotation support as well)?
We are also using DataAnnotations on our classes and this is a very important feature for us.
To insert DataFormatString on every RadGridView where you use a DateTime field doesn't feel like a very feasible solution when you could have it specified in just one place and have the controls automatically pick this up.
Another DataAnnotation we've got problems with is [Editable(false, AllowInitialValue=true)] which in the RadGridView has the behaviour that it allows still allows you to edit the field but when you're done editing you get a validation error telling you the field is readonly. Here again if I want the column to be readonly I'll have to specify it in xaml.