I would like to do a cake with pistachio for my sister's birthday this weekend. I am thinking of a WASC with pistachio filling, but I can't decide on a good frosting. Any suggestions?

Maybe just a regular buttercream? You could also add some almond extract but it can be overpowering so I wouldn't add too much, especially if you really want the pistachio flavor to come through.

Check out the Gourmet Flavors thread. It's like 300-some pages, but I know someone did a pistachio cake in that thread and said it was awesome. I think it was toward the beginning of the thread (maybe posted by pinkbox-not sure).

Here are a couple of recipes from the Gourmet Flavors Google Doc. It doesn't solve your buttercream problem, but maybe it will give you some inspiration for your cake and filling!

Pistachio-Cardamom-Caramel Cake- CakeGrlPA
1 box of white cake mix (I used DH)
1 cup of cake flour
1/2 cup of white granulated sugar (*I cut down the sugar because the coffee creamer and ice cream topping have lots of it)
2 teaspoons baking power
1 envelope dry pistachio pudding mix
1 teaspoon of salt (*If using salted pistachios omit salt).
1 teaspoon of grind cardamom.
*Sift dry ingredients twice and set aside.
1 cup of sour cream
1 stick of butter (at room temp.)
1/4 cup of veg. oil
1/3 cup of cream cheese at room temp. (*I always add cream cheese to my WASC recipe, you can omit this if you want, I just love the richness that it adds to the cake)
1/4 cup of brown sugar
*Cream these ingredients in the mixer for about 3 mins. or until completely blended together.
4 large eggs
1 tablespoon vanilla extract
Lorann pistachio oil flavor
*Add these ingredients to mixer until blended with the other ingredients.
1/2 cup of vanilla caramel coffee creamer
1/2 cup of caramel ice cream topping
3/4 cup of buttermilk
*Mix together all liquids and alternate with dry ingredients into the mixer.
1 cup of chopped pistachios (*dont forget the pistachios! add them to the mix at the very end)
Of course you can always do your normal pistachio-cardamom version of the WASC cake instead of my hybrid: pistachio-cardamom-caramel, but all I have to tell you is that this recipe is absolutely out of this world.

Pistachio Cardamom - MacsMom
2 pks white cake mix
1 1/2 c (8 oz) pistachios, finely ground
2 1/4 c cake flour
2 c sugar
1 tablespoon ground cardamom
1 tsp salt (omit if nuts are salted)
2 pkgs pistachio pudding
2 c pistachio ice cream, melted
2/3 c water
6 eggs
2 c lemon yogurt
1/4 tsp vanilla extract
1/2 c butter, softened

Pistachio Filling - Dancingcakes2008
Use pistachio instant pudding and replace the milk with heavy whipping cream.