So are the impulse engines using actual thrust to move the ship, or are they just sub-light warp engines like Rick Sternbach suggested over on Doug Drexler's blog that one time:
A careful inspection of the Nebula-class model reveals that it has no visible impulse engines. In a recent entry on Doug Drexler's weblog, Rick Sternbach indicated that he envisioned the ship's impulse drives as being hidden behind baffles to scatter their signature for stealth reasons. He added that "modern" Starfleet vessel impulse engines operated on little conventional rocket thrust anyways, being more of a sub-warp subspace drive.
Well, as long as it's called "impulse", it's not warp.
They simply forgot to add red glowy thingies on the Nebula class model. I really doubt that Sternbach "envisioned" it beforehand.