I didn't get any keys yet, but keep checking your Battle.Net account
every day. If you see that Diablo 3 Beta has been added to your list of games, then you got the key. Otherwise, keep checking every day.
They are choosing people randomly based on specs.
This does not mean that better specs = better odds. They're testing a huge range of machines, so don't worry if your PC sucks.
GOD I WANNA GET INTO THE BETA.