The method of registering to vote has changed since the Independence referendum last year, and although people who were already registered to vote were supposed to be carried over into the new system, there seems to be some doubt as to whether that has occurred.
Two MSPs have had to re-register and supply documentation within a week proving they are who they say they are, despite being registered at their respective addresses for about 30 years each. I went through the registration process this morning and was told that I was not on the register, despite having been registered at my current address and voting in every election for over 20 years.
There seems to be some doubt as to whether its a genuine issue with registration or poorly designed systems that are causing a degree of false alarm. However, either way, I figure better safe than sorry - make the effort to make sure you're registered.
You can register online, and read the UK government's information about the change.