Google engineer Adel Saoud said the following in a [now deleted] Google+ post: "Just got Ice-cream Sandwich update on my Nexus S & it's looking great :)."
Thus far, only Europe has seen Ice Cream Sandwich, with the Samsung Galaxy Nexus release over there. Those in the U.S. are awaiting the release of an LTE-supporting version on the Verizon Wireless network.
Ice Cream Sandwich is bound to come to many of the higher-profile recent releases, but not all of them. Those who can't wait can look to the developer community. Since Google released the ICS source code, folks have been working on custom versions.