The studio announced that as of today, several of Star Wars: The Old Republic servers have been locked. Locked servers are still playable, and inhabitants can create new toons; however, no new players can roll a character there. These source servers will eventually be merged with destination servers this summer.
BioWare's move to merge SWTOR's numerous servers follows a post-launch drop in subscriber numbers. Players on source servers are encouraged to manually transfer to destination servers before the game does it for them.
[Thanks to Paul for the tip!]