The Nexus name doesn't mean as much as it used to. Not all Nexus devices offer a pure Android experience - the Verizon version of Nexus will not have Google Wallet. In term of timely updates, the HTC Sensation received ICS update in March 2012 while the Sprint Nexus S 4G was not updated until early April.

So take that for what its worth. Don't be surprised if Google ends up allowing carriers and manufacturers to provide more customization with Nexus phones and be more lax with update timelines.