Sprint says it's beginning to rollout an update for the Nexus 5 on its network. The update enables Sprint Spark on Bands 26 and 41, along with providing "miscellaneous Android updates." The Android build number to watch out for is KTU48F.
It's likely this is the previously mentioned KitKat maintenance release, which will fix camera bugs and other issues — presumably that will warrant a bump in version number up to Android 4.4.3. Since the Sprint Nexus 5 is the same model as the GSM unlocked device, we could be seeing this new Android version across other Nexus devices before too long.