Pre-ordered and pre-loaded this last night on steamAlthough now it does mean I have two copies of DiRT 2 and can't gift my free copy from steam because my original copy was a hard one. Oh well.

I'm reasonably sure you could give away the GFWL CD-Key of your Steam version with your disk and it would work.
As you've already activated Dirt 2 on your GFWL account, I presume you didn't have to enter a new key when you ran up the Steam version?

Can we add friends so we can try to beat each others times on the stages?