Wow, I'm not an overly big fellow (6'2", 195 lbs) and usually conceal carry wearing shorts and a t-shirt; and I have no problems carrying my XD using a simple Uncle Mikes neoprene IWB holster.
Actually, I find carrying my smaller pistols (the aforementioned PT709, PT111 and PT 738) to be more difficult due to their smaller size. I've also found my 1911s to not be easy to conceal carry due to their heavier weight.
I am actually expecting the XD-S to be more in the $650 range simply due to the current prices of the XDm line. I wouldn't even bother with the BX as I've never found decent prices nor well-cared-for pistols at all the AAFES that I've been to that sell firearms. I will either order one online (I have a FFL bud who will do the transfer for free) or pick one up at a gun show if I find one at a good price (<$650).
As for this "trend," I believe Glock came out with a single-stack .45 ACP sub-compact with their G36 way back in 2000; but I don't think that particular pistol is as small as the new XD-S. The S&W Shield is nice and closer in size, but not available in .45 ACP so that's not an option.
Building such a small but usable .45 has been a real challenge for Springfield, and there was a lot of secrecy involved from what I have heard; but rumor is that it is a remarkable pistol and unbelievably manageable considering how small it is and the round it uses...
Cheers! M2