It's true, but not in the sense that most people would expect. It takes an unusual (at present) form factor of SSD; the standard 2.5" drives that are sold seemingly everywhere won't fit. Samsung and SanDisk produce the SSDs used in the rMBP, but I wasn't aware that users could purchase SSDs from anywhere besides OWC if they wanted to do the upgrade themselves (which is probably what xShane was getting at).It's not some what true, it's true lol.
Samsung, SanDisk and OWC Makes SSDs that fit. Just because tons of manufacturers don't make SSDs for doesn't make it "somewhat" or "partially" true. It's true, literally.
We're not Apple salespeople here. There's no point in hiding the upgradeability of the SSD, but the rest of that information is critical. I see it like selling someone a car with a diesel engine in an area where most gas stations don't carry diesel. Sure, it's a car that takes gas, but make sure you know what you're getting into