I had the same problem when Crisco took out the transfat and I was using the Wilton Class Buttercream recipe. No matter how long I let it sit or how much blending I did I couldn't get rid of it.
I recently used indydebi's buttercream recipe and it fixed the problem 
You can find it in the CC recipes.
Sounds like it's the salt to me, too.
My Wilton instructor said to mix the salt in with the water, make sure it's disolved or it will do that in the frosting. I doesn't matter if the butter is room temp, if the salt is not disolved in the liquids first, it won't disolve and will make it spotty.
Regarding the Wilton recipe, have you used their new recipe? They adapted it this year for the new transfat-free and it worked great for me.
I didn't know that. I looked on the Wilton site for it but couldn't find it.
Would you happen to have a link to it?
Thanks
Okay, let's see. I just made it so I hope I can remember. It was in my Course II/III book.
This is for stiff.
1 c. shortening
2 T. water
1/2 t. vanilla??
Cream.
Add 1 T. meringue powder
4 c. (1 lb) powdered sugar.
salt (optional)
I do believe that's it. Am I missing any major ingredient?
Also, I just read on a Wilton forum that Crisco suggests adding cornstarch. Maybe that will work?
You are right ...I think is a teaspoon of extract..They are recomending now using milk to bring the fat to the recipe since crisco took it away. They have a new recipee with the butter in it too. Is in my website in the recipe section. I havent try it yet!
Edna 
I also had this same problem with my frosting! I also was use the Wilton recipe in the course book but I do not use the transfat-free Crisco. I use the generic store brand and have never had a problem until recently. My instructor suggested using milk instead of water in the recipe and so far it has worked! No more spots! So you may want to try that also. Good luck! It's very frustrating having speckled frosting!!
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