I have the subcompact 3" XD in 9mm - I like it, but it probably doesn't conceal much better than the 4" model. The main issue is the width of the slide - it's pretty wide. I have a simple Uncle Mike's pancake OWB holster (Sidekick #1) which accomodates my ultra-compact 1911 3.5" barrel, and my Taurus PT940 (a sort-of beefy Beretta) but I just got a (size 16 Uncle Mike's pancake) for the XD.
I am thinking of getting the XD with a 3" barrel, probably also in 9mm. What do you folks with the subcompact XD think of the gun with regard to concealed carry?