Since Galaxy Nexus, which already had more than good enough hardware to run KitKat, isn't receiving an official upgrade from Google, you won't be seeing one for Nexus S either. But that doesn't mean the ROM community will just leave it behind. In fact there is already a KitKat ROM for it that's said to be "running pretty fast".
The Nexus S could very well be the poster child for KitKat – it has only 512 MB of RAM (exactly how much Google is saying is required to run KitKat smootly), a 1 Ghz single core ARMv7-based Cortex A8 processor, and 16 GB of storage. So in theory, there's nothing wrong with the device, and there's no reason why it shouldn't run KitKat well, if what Google said was true.
XDA member cn.fyodor wanted to prove that theory, and even though he's only been working for a few days on a KitKat build for his Nexus S, the ROM seems to be working pretty well. The ROM is still alpha though, so some things like some Google Apps aren't working properly, you'll be able to make calls, browse the web, and run any other 3rd party app (unless it has issues with KitKat itself).
It's really too bad Google themselves and also some of the OEM's didn't take it upon themselves to push KitKat to as many of their low-end devices as possible to show just how much they care about their customers. They wouldn't even need to update pre-ICS devices, since that does involve a huge amount of work. But they should've at least upgraded all Android 4+ based devices that launched from 2012 and beyond. All devices that have ARMv7 processors should have at least 512 MB of RAM, and enough storage to be upgraded to 4.4.
Unfortunately, most OEM's don't care enough about upgrades, after they sell their phones, especially at the low-end. KitKat will most likely land on new devices that come at least with a single-core or dual-core Cortex A7 processor, 512 MB of RAM, 4 GB of storage and a 480×320 resolution. These phones won't arrive until next year, and they are likely to cost even as little as $50-$70, off-contract, helping Android gain that next billion of users.