To sort a picklist in reportbuilder, when working with a model, you can use the following workaround :
* add the field twice
This is pretty daft : you can't just sort the list. In t-SQL, it would be sooo easy, but on a model, your hands are tied. Once you add that same field a second time to a query : disco, it's sorted.
Took me a while to figure out that it was actually the addition of a field a second time that sorted the thing.. and not any function that was applied to the field.
Gotta love RS.