When it comes to treats like ice cream, I look at the carb content and not necessarily at the fat content. I've been using Edys sugar-free when we want a treat like that, but at an average of 10 grams effective carb per 1/2 cup, we don't do it often.
Another option would be to make your own. Recently, one of my daughters had strep throat so rather than let her chow down on regular ice cream, I took frozen strawberries and put them in the the food processer, added 1/4 cup of Splenda, poured heavy cream over it and processed until smooth...just like soft serve ice cream but with a lot less carbs. You could do the same thing with frozen raspberries or blueberries.
I've also recently discovered Atkins Endulge ice cream at my local health food store. It's pricey at $5.00 American for 4 little cups (1/2 cup each), but it's quite tasty and has a low effective carb count as well.