I've tried out a couple of different kind of smoothies:

I did one with 100 grams of plain yogurt (as full fat as I could find) (12 grams of carbs) Some raspberries, and a scoop of vanilla whey protein powder--it tasted OK. Not very sweet...maybe next time I will add some stevia. It was around 20 carbs for it though, which for SP would be about right, for PP, it is a little high at my level.

I did one from the Protein power book a while ago, which was vanilla whey protein powder, 1 ounce heavy cream, 1 packet Splenda (I used Stevia) and 1 cup ice. (I added about 1/2 cup frozen peaches to it). Wasn't too bad--it was around 8 grams of carbs.

I would highly recommend Linda's Low Carb site for ideas:
[URL=http://users3.ev1.net/~fontlady/]
Under her "Snacks" section she has several different smoothie-type recipes.
Most of them use Splenda or Stevia or something to sweeten them, but that would be at your discretion of course...