If you wear a polo shirt then I think there are only 2 realistic business casual options - the ankle holster and the wallet holster (a front pocket holster is too obvious when wearing slacks -- the weight will pull down too much.) Here are some pictures:
However, I don't think they are too comfortable to sit on. They obviously don't bend and conform to your butt like a regular wallet will. Note that in both cases the pictures show someone in jeans, which are a thicker and tougher material, but I think this would be fine for nice dockers.
I like the full holster pocket option. That, and a portfolio type case are my two primary options so far. A holster for when a portfolio is not carried.