A Very Brief Historical Perspective
With the uptick in popularity of aiwb lately, there is a perception that it's relatively new. Nothing could be further from the truth.
The ubiquitous Milt Sparks Summer Special, first developed by Bruce Nelson while he was working undercover, was originally an appendix holster (for a Colt Commander). That's right, the granddaddy of IWB was an appendix rig. AIWB was used extensively -- with and without a holster -- by many plainclothes and UC law enforcement officers for a very long time.
Its popularity waned with competition, particularly in IDPA where holster position was dictated as being behind the right-left centerline of the body, and in IPSC/USPSA where concealment was completely abandoned. (It's worth noting that many USPSA competitors still wear their guns forward of the hip)
Among certain crowds, though, the aiwb concept never lost favor... mostly among people who operate in non-permissive environments.
While there's a perception that aiwb's popularity spurt happened recently, there are threads at places such as Total Protection Interactive going back more than five years discussing it in detail. Those threads credit people going back years further.
If there has been a major change in recent times, it's the resurgence of interest from holster makers who now offer far more practical, concealable, and comfortable aiwb holsters... in particular, they offer aiwb holsters that work with the kind of larger (G19 and bigger) ccw guns that are popular among the internet forum crowd.