she recovered pretty quickly from Alistair's torture..maybe they have the ability to heal such wounds quickly..
but it would have been a nice hint in the first episodes of s4, when Ruby opened the door, just a little bleeding cut on her forearm or so..

Actually, I've been thinking about this too. And I don't think demon meat suits heal at all. Remember Meg? They threw her out of a window months before they exorcised her and she died from those wounds and the gunshot inflicted by that other demon. Or maybe it depends on the demon- those that don't care about the host won't bother to heal it?
Another explanation is that Ruby used her extra witch skills (she's a powerful enough witch to throw both demons and angels off their trail) and healed herself that way.
Also, I'm glad they didn't give us any real hints, especially in the beginning. They talked about it but you had no real idea what it was they were talking about. I loved the absolute shock I felt discovering that that was what they'd been talking about. It's something I'm glad I didn't see coming!

I'm not sure but I think that Ruby tends to wear long-sleeved shirts or jean-jackets and that's how she could've hidden her wounds.However,in the sex-scene I don't remember seeing any wounds.Maybe Sam needs demon blood only in special occasions when he feels weak...

Unless of course I missed something, I'm not sure if that was implied; if it was, did it leave no doubt? It seemed to me it was something relatively recent.

I think that demons can heal/hide wounds while they're possessing the body, but once they leave all those injuries reappear.