"All these ships were called Enterprise," Decker said to Ilia. The mysterious ship with the hoop-shaped warp drive has hovered around in non-canon Trek lore ever since. It even made a brief appearance on Enterprise in the episode "Home." (Look for it toward the end of the episode Archer has his last conversation with Soval. On the wall of Forrest's office, behind Archer's head, there is a picture of a ship with the distinctive ring structure.)
When we finally saw ships from the Vulcan fleet on Enterprise, they also had hoop-shaped warp drive. The suggestion seemed to be that this was a Vulcan design that human tried to emulate, then gave up on, returning to Zefram Chocrane's twin nacelle design.
I recently came across this article which confirms that idea, albeit not from Star Trek cannon, but from Doug Drexler who designed the Vulcan ship in the first place. That was his basic idea behind the distinctive design.
You can check out the article here: http://www.starshipdatalink.net/art/timur.html [Link has been updated.]