bakechef
Executive Chef
I had to make a wedding cake for someone and did a lot of research on how to get a very smooth finish with buttercream because the bride didn't want fondant due to the texture/taste issue, even with homemade fondant. On the Wilton discussion board seasoned cake bakers gave the tip of using interfacing to get a smooth, almost fondant looking finish on buttercream. Frost the cake, let it set up until the outside is just slightly dried/crusty then take strips/pieces of interface cut just a bit larger than sides/top, lay it on top or against side of cake and gently smooth your hand over it. The warmth of your hand will smooth out any ridges in the icing and you can just peel away the interfacing. You need to use new, clean, never washed interfacing for this so it will be perfectly smooth without any creases. I just did a couple of small practice cakes, it was an easy technique to master.
Viva paper towels work really well for this too, since they have no pattern at all.