If you purchased an HTC EVO 3D with HBOOT 1.50 right out of the box, or you held off rooting it thinking Revolutionary.io was still solid for the latest OTA, you might think you've been stuck with a half-rootable phone.
It appears, though, that having S-ON only means the kernel image cannot be tampered with in normal recovery. There are methods of launching recovery from fastboot by which I have successfully flashed a kernel and a ROM together, but until now flashing a kernel alone has been a bit of a task.
Flash Image GUI will flash an image at a different level than recovery, cutting out the need to boot into fastboot then force a recovery load via command line. Evidently the trick here is the only time S-ON matters is at the lower/boot levels of the OS (either that or I'm reading the entire thing wrong).
You can download the program for free here, but consider showing some support by purchasing it from the Market if it works for you. It takes developers a lot of time and work to figure these things out, and Bawls Soda ain't cheap.
Try it and let us know if it works for you. As usual, make sure to create a Nandroid backup before doing anything root. Oh, and this app also works for boot logos, flashing recoveries, etc.