1. Fastboot oem unlock will unlock your bootloader and do a complete wipe of your device. As far as I know the only way for something to go wrong here is if you're device is low on battery and shuts off during the process. Either plug it in or have a "good" amount of juice left in your battery. To be 100 percent sure plug it in.
Before I get into oem lock IMHO you should only use this command for one thing only. Getting the device ready to send into motorola for the LGT upgrade or a another warranty repair or some sort. When done properly it is completely safe but there is no reason to do to go back to "stock".
2. Fastboot oem lock. Do not issue this command unless you are absolutely sure you have the correct software on your device. The correct software is what you get from the motodev site for your device. You need the official sofware images for your specific device AND REGION. If it is not released then DO NOT re-lock the device. If you have a OTA installed you will need to flash back to the motodev images first.
All this info is all ready on the forums in various threads already but I wanted to put in one place since I noticed quite a few users botch up the devices trying to update to 3.1 The last thing we need are a bunch of people bricking the devices and sending them in moto. I suspect if enough people do it they will end up changing their minds on the entire encrypted bootloader situation.