To make any prediction about tech is usually very foolish, but I'm going to make one anyway, since I think it's pretty reliable. Here it is: in five years, IPv6 deployment will be at at least 50%, with significantly more support at the home ISP level. All major sites will support it, as will Colocrossing.
Basically, at the end of five years, I don't think there will be any choice left. For the few major companies that don't support it yet because of economic reasons, by 2018 the economics will be do or die. Five years is my "margin", unless some brilliant IPv4 conservation method is found. (No, carrier grade NAT is not "brilliant"). What do y'all think?