Just had some of the Harpoon "Single Hop ESB" (http://www.harpoonbrewery.com/index.cfm?cdid=145607&pid=28516)
The single hop in question is the Delta Hop, which is a Cascade/Fuggle hybrid. It was developed by Hopsteiner, but appears nowhere on their website.
There are definite early citrus notes, and a noticeable earthy character, but both seem subdued compared to what you'd get in a brew with both hops. Can't find anymore information about this hop outside of Harpoon's website, but I'm definitely keeping my eyes out for it.