Unlimited respecs works great in Guild Wars since you have a fuckton of skills at your disposal and just as many viable builds to choose from. Also, there is no such thing as discovering "the ultimate build", since for every single build that has seemingly gained that title, another build is soon discovered that can counter it effectively. Thus the game encourages you to be constantly changing, adapting, experimenting. However, I doubt D3 will go in that direction; somehow it doesn't seem to me that it fits with the spirit of the series.
WoW's respec system, which would probably be the main influence behind D3's potential respec design, is geared more for allowing the player to switch from a PvE-focused build to a PvP build and vice-versa. There is room for experimenting with crazy build ideas, but there's very little of it since Blizzard has shown to be determined to force you to play your class in two or three specific ways in order to get the best performance out of it. D3's system would probably work the same way.
For those of you still vehemently opposed to any kind of respec for fear that it is an instant character hermaphrodite-er button that allows noobs to pwn pros, keep in mind that, as Destroid said, skills and attributes only comprise half of your build (and I doubt respecs will allow you to respec anything other than skills). If you think grinding more than one character of the same class as the one you already have at level 99 is fun, then you are simply a masochistic, unimaginative fucktard. Why don't we suggest to Blizzard that they remove those sissy waypoints and town portals? Why not remove healer NPCs and that stupid "free item identification" from Deckard Cain? Hell, why not make it so that you can only ever have one character saved at a time?
P.S.: for clarification, the way respecs work in WoW is that you get all your talent points (the rough equivalent of skill points in Diablo) back by talking to your class trainer at a big city and paying 1 gold, which can be a substantial amount for mid-level or otherwise poor characters. The second time you go to respec it costs 5 gold, then 10 for the third, 15 for the fourth, and so on in increments of 5 gold up to a maximum of 50 gold each time. For every month that you don't respec, the cost goes back down by 5 gold until it goes back to 1 gold.