In a smoothie you could probably get away with using yogurt, and maybe cut down some of the liquid ingredients to make up for it (I think yogurt would be more liquidy overall... that's a guess). But in a smoothie it's not gonna be lumpy at all anymore so I don't think that should be an issue... unless you can't even stand to look at it as you scoop it out of the container. =P
Ricotta could be used in some recipes instead, if you're ok with that texture. In some things you could maybe use goat cheese.
If you eat soy, you could use tofu in some cases.
If you post an example recipe we could probably help more. =]