This addin for Nintex Workflow 2010 adds the ability to configure SharePoint groups whose members are allowed to delegate workflow tasks that have been assigned to other users.
Please refer to the Nintex Workflow 2010 SDK for more information, specifically "How to > Create a custom task form".
UPDATED: 16 January 2012.
I absolutely second that question! Would need that solution in SP2013 too. It's a pity that this is not followed up and ported for SharePoint 2013 (and 2016). Approval Actions in Nintex Workflows are so powerful, but when it comes to the flexibility by delegation the means provided are rather for admins.