First of all, players will be able to move servers at will for a while after release so that they can play with their friends. Since the game is launching with the ability to switch servers, it should come as no surprise that, in the event that ANet needs to open new servers, players will immediately be able to transfer servers (for free if you're coming from select crowded realms). People also have the option to "guest" on servers, allowing them to play with friends from other servers (unless you want to enter WvWvW with them, in which case, you will need to switch servers). That still doesn't fix the "Oops! I created on the wrong server and have to pay for a transfer" situation, so Mike said that ArenaNet will have a warning up after release so that new players will understand what might lock them into a server. However, people playing the beta test will notice that the warning has already been implemented!
So I asked Support about it - asking if:
So at least that's something off my list of concerns for launch.