It all has to do with how your FFL calls it in and designates it.
When we transfer a stripped and previously unassembled lower receiver to a customer we transfer it as exactly that: A "receiver" or "other" on form 4473 box 29. Most stripped receivers we transfer are marked "multi" in caliber and have no designation as far as being a pistol or rifle, so that it's really up to the purchaser as far as how they're going to use it. ATF does not require that pistol lowers be marked as "Pistol", although some individual States do have laws requiring the registration and marking of pistols. So it's key to know the laws in your individual State to make sure you're within compliance. NFA regulations simply state that pistols cannot have stocks or vertical grips attached, either of which would constitute a Short Barreled Rifle.
The key thing here is to make sure you know your Federal, State, and local laws. Generally, if you put a pistol buffer tube on the lower receiver then you've made it so that the receiver cannot accept a stock, and as long as you don't have extra vertical grips laying around then you're pretty much good to go. Local laws may vary, so it's key to know if yours require anything further as far as registration or engraving. It's really not that complicated or scary though, and Justice is right that AR pistols can be a lot of fun.