Google may not have officially pumped out Ice Cream Sandwich for the Nexus S 4G yet, but that won't stop you from having it. We've got two custom ROMs in the forums that are dripping with ICS goodness. If you're a fan of the stock Android experience, you can get a fully working AOSP 4.0.3 ROM for your 4G, with just enough changes from standard to make everything work well from Beezy. It's like having a smaller Galaxy Nexus in your pocket, with face lock, beam, and everything else you've seen in videos and reviews.
If you want something a little different, fergie716 has ported over the latest ICS MIUI ROM to the Sprint Nexus, with everything good to go except for Wimax (which you'll never see working on MIUI). It's also Android 4.0.3 under all the candy, and offers a very different experience that everyone should try for themselves at least once. It's subtle, beautiful, and a nice change of pace.
Of course, nobody here will blame you if you just want to wait for Google to get the official 4.0.3 build pushed to you. But if you like to tinker around, there's no need to wait. Check 'em out.
Thanks, devs, and thanks Paul for the tip!