Same problem here with boot looping, but it seems Skype and 2.3.6 are allergic to each other. I went back to the states for the Thanksgiving holiday and bought a prepaid T-mobile SIM card and the first thing the network wanted to do was upgrade the firmware so I thought great-make use of the faster connection while I'm here...since then it's been the blues. I've uninstalled Skype and that's reduced the number of times it reboots.
Yet, it still reboots asking for the SIM pin number every time.
It's annoying when at 3am you hear the start up melody from the phone as it sits on the nightstand...Argh!
Going back to the 2.3.5 firmware the S2 worked really great for me with it.