5.4 hit the PTR. Don't go to MMO-Champion if you don't want some spoilers...
It looks like they're working on merging low pop servers. I'm not a big fan of it though. It sounds like they'll be creating a new server group just for low-pop servers. You'll be able to join guilds that are a part of that server group, trade, and join guilds. This is going to get horribly confusing real fast with servers, virtual servers, and battle groups all merged through different systems allowing different things. Good to see they're working on it though.Patch 5.4 adds Virtual Realms, which will essentially merge lower population realms. This will allow you to keep playing on your current realm, but do arenas, raids, and dungeons with people in your realm group, as well as use a merged Auction House and trade with other players in your realm group.