Google has announced that it's rolling out Android 4.0 to GSM Nexus S devices "over the coming month," but it should surprise precisely nobody that Android users have figured out a way to download and manually install the update immediately. Android Central details the steps, which follow the tried-and-true Nexus method of downloading the full ROM image to your desktop, renaming it to "update.zip," transferring it to your device, and then hopping into the bootloader by holding down power and volume up from a powered-off state. The ROM download comes straight from Google, you'll be pleased to hear. If those instructions sound daunting, Google will get you a simpler over-the-air update soon enough, but if they don't, the full instructions and download links are in the source links below.
As with the update Google is rolling out OTA, this one only applies to GSM variations of the Nexus S, so Sprint users will need to wait a bit longer. This ROM is actually the latest and greatest 4.0.3 version of Ice Cream Sandwich, which Google has said will be the "base version" of Android 4.0 going forward.