Yep, mine were the same Manta: they could support their heads almost from the moment they were born. I remember taking a photo of my third baby at 3 days old and posting it for my June Baby Buddies to see. One member said "He looks like a 3 month old!!!" (the way he was sitting up straight on my lap).

You're right: you can just tell when they aren't going to sleep in a certain way. I rarely sleep on my back myself. The idea of it makes me feel uncomfortable and "exposed" I need to be able to curl up a bit.