Just for general information for anybody who may be wondering, the version of the game purchased from Direct2Drive installs fine. (I used the C4 Data file.) I got some rather peculiar messages during the install process, but after all the dust had settled the game started up and works. (No game icon, though.)
So far, I've been able to ramp up the graphics and sounds settings to their highest levels with no problems, though I can't seem to increase the resolution without causing the game to become a blank screen (but with sound) at launch. The intro movies work fine. The Tutorial, skirmishes, and replays seem to work fine too. Haven't tried the campaign yet.
Can the game's "Details" page be updated to reflect that the Direct2Drive version is known to work?