I had this problem today and noticd when it did the reboot the first Sprint screen would show but it would not display the second part saying "together with nextel". I held down my power button on the next reboot and it ran fine. I believe this has to do more with trying to load a program from the SD card before the WM is prepared to have a startup program run. I don't believe this is a 3rd part software issue.
Had this happen to me as well after loading the HTC 5 tab plugin with color icons. I did a hard reset and loaded the plugin without color icons. It now does the "loop" once when I soft reset, then boots.
Had it happen to me after installing imov messenger. That was the only time I've had it happen and have installed quite a few other apps. Had to remove the battery to get it to stop.