In the movie A Man Called Horse, Richard Harris (AKA Original Dumbledore) recreated the feat using a prosthetic chest. Around this same time, or slightly earlier depending on who you ask, the modern primitives movement was beginning to take off, and people started doing suspensions for spiritual reasons, and from there quickly branched out into non-spiritual, meditative and the why-ask-why-it-looks-so-cool type reasons cited today.
How mainstream has it gotten? Well, today you can go to Suspension.org and search for groups in your area and probably find a group to join. In Texas, where I live, there are like a dozen groups. They have them in Anchorage, Alaska and Grand Rapids, Michigan. Articles have appeared in newspapers and magazines around the world in increasing frequency. Model/Performance artist Miss Crash, who has been suspended as the opening act for the band Jane’s Addiction has been featured in Spin and Rolling Stone.