LOL Ok... I know its a neat trick and who ever thought it up should have patent it because he would have made a fortune!
The amount of times I get asked this question is not even funny.
As soon as they get to word "reflective" I say:
"STOP!......" and go on to explain how this illusion of "reflective floors" in Second Life is created.
So, there is no such thing as true reflection in virtual worlds or game engines period.
The closest thing I have seen is in The Sims3 and it took me by surprise actually, one of the little sim NCPs walked in front of what looked like a window and he was reflected back to perfection.. must be some kind of "screen within a screen" trick.
Anyway, in Second Life is all trickery, clever, prim expensive trickery.
All the builder does is duplicate the walls, a few of the major pieces of architecture within a room, spin them all upside down to create a "mirror" effect. Then they make the floor prim semi transparent (so you can see the duplicate walls and objects mirrored below) slap any marble or wood texture on the floor and set the shine to high in the texture edit options... TADA!
Now, if anyone else asks me I can refer them to here :)
Whats really sad is this morning the lady who asked me said she had asked several store owners where they got their "reflective textures" from and none of them would tell her. (that's just saaaad!)
I wouldn't mind but it's hardly a new phenomena! It's been used around SL for at least 2 years now.
Here are some links to and about how to do it: