morfic at XDA-Developers has managed to overclock Nexus S to 1.2 Ghz!
Warning: Rooting/Overclocking your Nexus S may cause unseen damage to your handset. All of this is done at your own risk and we take no responsibility for any damage you inflict upon your precious Nexus S.
To be able to overclock your Nexus S to 1.2 Ghz you need to have a rooted Stock Android 2.3 ROM, the instructions to which can be found here. Even though the developer has listed this kernel as a Stock Gingerbread ROM only kernel, a daring user tried this on the MoDaCo ROM and reported it to be working just fine.