I hadn't actually seen anything official stating that the plan for the movie was essentially to re-create the first movie, so I went looking for it.
This article is what I found. The key quote is this, I think...
"The writer added that he believes Mass Effect has a mythology as rich and deep as those of Star Wars and Lord of the Rings, but that they are focusing solely on the first game for the movie version."
To me, that doesn't necessarily translate into telling the whole story of the first game. It could mean taking one portion of the first movie, say, the discovery of the Reaper threat, and building around that. It could mean just exploring the adventures of Shepard as the first human Spectre. It could mean any number of things, really.
Has anyone confirmed that the plan is to tell the whole story of ME1 or have we all jumped the gun here?