Grargar: 3 months exclusivity for X-Wing vs TIE Fighter and X-Wing Alliance. It was going to happen, sooner or later. Interestingly, the descriptions of TIE Fighter and X-Wing don't seem to mention the Collector's CD-ROM versions.
And exactly 6 months exclusivity on the first two X-Wing and Tie Fighter games. GOG release date 10/28/14 and of course, today is 4/28/15.
Berggeist: I'm not dropping money to find out but as far as I can tell it's just the 1998 remakes in the XvT engine and the 1993 dos originals. The forums so far don't have much in them, but it includes concern about which edition they're getting, curiousity about the difference compared to the gog edition (as in concern it wouldn't be stable as steam doesn't fix up old games), and people complaining about the price.
The description of X-Wing on Steam states:
"This product includes STAR WARS™: X-Wing and all Tour of Duty expansions: Imperial Pursuit and B-Wing in both 1993 (DOS) and 1998 (Windows) versions."
So, yeah, definitely don't have what GOG put out today.
As far as the Steam version "not being as stable", I'd be willing to bet that what is on Steam is specifically the GOG release. The price of the initial exclusivity on GOG was probably GOG doing the work to make the games compatible so that they could later be released on Steam.