Tom Warren over at WinRumors is demonstrating a Windows Phone OS issue whereby an SMS message can force the phone to shutdown and disable access to the device's messaging facilities upon reboot. The issue can also be triggered by messages received from Facebook chat or Windows Live Messenger and appears to be device agnostic, affecting the current 7740 build of Mango as well as the RTM build 7720. The only way to fix the Messaging Hub is to wipe and restore the device.
The content of the malicious SMS message is not specified, presumably because WinRumors and Khaled Salameh, the person who discovered the issue, are now working directly with Microsoft to disclose the specifics of the attack. Although SMS issues have created vulnerabilities that could be exploited on other mobile OSes, WinRumors says that the bug affecting the Windows Phone Messaging Hub does not appear to be security related.