Well well...I see our little announcement has sparked the rampant speculation, again!
First off, it's worth pointing out that we didn't choose the Imperial Guard simply because we had already included a few IG units in the original DoW. In fact, we've thrown out the IG units from DoW and made new ones that are more appropriate for the Expansion (so, no reuse at all!). We're creating new effects, voice work, the whole nine yards.
And as Tranj pointed out, our work on the Expansion does not preclude continued support for DoW. In fact, from your point of few, it guarantees continued support for DoW because an Expansion is in effect an additional investment in the original on THQ's part.
As for the 3 screenshots listed above, the Baneblade concept piece is not from a pre-rendered movie. It was used internally as concept art to illustrate the massive tank firing its weapons. Pretty effective, isn't it?
