The way it works is that ArenaNet will be reserving all current Guild Wars names for the launch, and players attached to those names can then swoop in and collect them from the start. Three caveats are attached to this process, however: You have to log into Guild Wars sometime in 2012, you have to claim your names in Guild Wars 2 by launch day, and you can't secure Guild Wars 2's single names (as Guild Wars forces you to make two-word names).
The studio says that there are 20 million two-word names in use at this point, and we suspect that list will grow much longer after today. These names can be reserved at any point from now until whenever ArenaNet decides to compile its master list. After launch, ArenaNet promises to free up the unclaimed names for others to take.