quick review of web recipes confirm what I'd do
make regular icing recipe (I use IndyDebi's), but sub in peanut butter at ration of at least 1:1 for the fats or better yet 2 parts PB to 1 part fat.
so if recipe calls for 1 cup of fat (butter or shortening) then it would be at least 1/2 cup each or 2/3 PB + 1/3 fat
I would use butter and not shortening to get smoother, softer consistency as PB is already fairly stiff at room temp.
of course this one also looks deadly delicious:
http://www.foodnetwork.com/recipes/ina-garten/chocolate-cupcakes-and-peanut-butter-icing-recipe/index.html
(hmmmm ok...now this has me thinking -- and just how much honey would it take to make honey PB frosting?)
THis one was recently posted in the recipe section, though I haven't tried it yet.
http://cakecentral.com/recipes/15065/whipped-peanut-butter-buttercream
I've made this recipe for a friend and he told me that next time keep the cake and just give him the frosting...... lol
Peanut Butter Buttercream Icing
1/3 cup solid vegetable shortening
1/3 cup butter
3/4 cup peanut butter
3/4 cup milk
1 tsp. pure vanilla extract
4 cups (approx. 1 lb.) confectioners' sugar, sifted
Makes: About 3 cups
Instructions: Cream shortening and butter with electric mixer. Add peanut butter, milk and vanilla. Gradually add sugar, one cup at a time, beating well on medium speed. Scrape sides and bottom of bowl often. When all sugar has been mixed in, icing will appear dry. Add milk and beat at medium speed until light and fluffy.
Keep icing covered with a damp cloth until ready to use. For best results, keep icing bowl in refrigerator when not in use. Refrigerated in an airtight container, this icing can be stored 2 weeks. Re-whip before using.
I had a student last year just add 1/4 cup creamy peanut butter to her buttercream and it was delicious! 
I know it's not really what you're asking for, but I made a peanut butter cake and used chocolate ganache and a buttercream based fudge filling. It was soo good, tasted just like a peanut butter cup! The peanut butter cake and fudge filling recipes are in here.
http://docs.google.com/Doc?id=df4f9hbq_46cs9f28fs
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