Users on XDA-Developers have discovered after they received the firmware update which brings internet sharing to their devices, they have suddenly developed a dynamic MAC address, which is of course a major no-no and possibly illegal.
The Media Access Control address (MAC address) is a unique identifier assigned to network interfaces and is normally unchanging, except for spoofing done by hackers to circumvent security, such as MAC address filtering, where hackers change their MAC address to pretend to be another device.
It seems however after the HTC firmware update, if you want to change your MAC address on a Windows Phone all you have to do is simply reboot. The bug does not require you to actually use or activate internet sharing.
Besides being illegal, the problem can cause issues for users who are in an environment where MAC addresses are used for security, access control and to assign static IP addresses. With the address ever changing none of these measures will work very well.
Some XDA-Dev members have reported that HTC will be addressing the issue tomorrow. Till then, if you do rely on a static MAC address, it may be a good idea to decline the update when offered.