Sony has released a new firmware update for PS3. This follows a number of complaints that the previous version was bricking some consoles by totally removing the XMB. Obviously, you can't navigate through PS3's functions without the XMB... Whoops.
If you're one of the unlucky few who downloaded version 4.45 and lost your XMB, you can update your PS3 by downloading 4.46 to a memory stick, rebooting your PS3 in Safe Mode, and installing the new firmware from there. Here are the instructions on using Safe Mode via the official PlayStation website.
What is actually in the system update? Not much. General improvements to make your PS3 run more smoothly, and the option to turn off Trophy notifications.