As we explained yesterday, now that the Ice Cream Sandwich SDK has been released, we're about to see a deluge of SDK-based ROM ports. We're not going to cover them all here, but will will cover the first to come out of the Android Central Forums. Beezy's been hard at work, and you'll soon be able to enjoy the fruits of his SDK labor on the Nexus S 4G. Things are very much still a work in progress, but you'll soon be able to be the first on your block to say you have some form of ICS on your phone.
Source: Nexus S 4G forums