By thunder, you're right. Weird...it just looks odd for the engines to be dark, even at warp. BTW, here's a good shot of what I and CptBenSisko were talking about on the Ent-A:
EDIT: Now I realize why it looks odd that the impulse engines are dark at warp, when the Enterprise-A warped away at the end of IV, her impulse engines were still active.