My guess is that you get rested xp *up to max level*
a level 79.5 will have 0.5level of rested xp max.
a level 80 will have 0 rested xp.
This is my understanding.

Not sure how they could calculate the rested xp anyway, as the current xp shown for level 80 has nothing to do with what you'll have to go through when cata comes. I guess they could do a % and grant the rested xp proportionaly, but my guess is that you won't be able to.

in any case, try it and tell us how it goes :-)
If you're right you'll get 1.5 level of rested, if not .. well, you'll have to kill a couple mobs to get to 80 first hehe

I don't really care either way, all my important toons are already 80