An internal testing Ice Cream Sandwich ROM for the Samsung Nexus S 4G has been leaked out of Sprint, containing a new software modem, bootloader, and a 4.0.4 build. Some are reporting this is the final version and we'll soon see an OTA for the NS4G, but that's speculation at this point. We do know that this seems to run smoothly, with everything working including the Wimax radio. It's build number IMM26, with a final kernel build and the ROM is signed with release keys which makes it pretty likely.
Of course the OTA for the Nexus S 4G can't come soon enough for many users, who have seen the 3G version of the Nexus S and the Motorola Xoom get their ICS builds pushed to them. We have to remember that Sprint not only has more network testing involved in their version, but the addition of Google Voice integration and Google Wallet means their release cycle will be longer. I'll go out on a limb and say Nexus S 4G users will see ICS released long before any other handset gets it, even if it's not imminent. We've seen extremely stable builds of Android 4.0 for the NS4G for a while now, and you know the developers already have their hands dirty (trust me, they are all over it) using tonight's leaked files to make things even better. If you're not the type who follows the manufacturers update path, hit your favorite ROM developer in a day or two.
If you're not the waiting type, visit the links below and try it on for size. Heed the warnings, and if you hit any snags visit the forums and holler!