Blizzard poster Belfaire has confirmed over at the forums
that the first wave of Beta keys for subscribers who have opted in have been sent out. If you opted in, you might want to check your inbox
for a Beta key as Blizzard has said that the invites are random
. If you're one of the lucky few, you just might be roaming the new (but not necessarily improved -- we hear there are lots of bugs, and not the Silithus kind) build of the Beta. If you didn't get a Beta key, fret not as Belfaire also mentioned succeeding waves but did not give an ETA. You can check out WoW Insider's extensive beta coverage
while you wait.