I can't understand how the SIM would be the problem. In my case, when the storage card was removed it would boot perfectly fine. Keep in mind that the SIM was never removed during the process which leads me to believe that the SIM is not the issue.
I've been having my SIM card for 6 years now and never had an issue w/ my phones before upgrading to WM6. Why would your SIM all of a sudden go bad after you upgraded? Troubleshooting is simple. Same Device, WM5, Sim Card, Storage Card - No Issue. Same Device, WM6, Sim Card, Storage Card - Reboot Loop. The only thing that has changed in the entire configuration is the OS. Thats the cause of your problems. If you revert back to WM5 your issues would likely disappear.
"You can get more with a kind word and a gun than you can with a kind word alone." – Al Capone