Thanks, everyone. Looks like it can be done in five moves, which is the minimum...

DraeKlae
1.d4 d5 2.Nc3 Nf6 3.Bg5 Nbd7 4.Qd3 c6 5.O-O-O

Conzipe
1.d4 d5 2.Nc3 Nf6 3.Bg5 Nbd7 4.Qd3 g6 5.O-O-O

Flier
1.e4 d5 2.exd5 Qxd5 3.Nf3 Bg4 4.Be2 Nc6 5.d4 0-0-0

...The next step is to research games where titled players (>2400) used these lines. I'll post what I find. - Mark

Ok funny the one thing i noticed about  the Carlsens weekend victory over Topalov was how early he castled so this answers your question i think

I just did it in six, in response to White castling king side in four:

there is no point of doing fastest 0-0-0

Mind you, though castling is a real important aim in the opening, amongst more experienced players, in certain variations, castling too early can in fact llow the possibility of your oponent unleash a vicious attack... So, i most lines i tend to wait tioll my oponent shows his cards first... Even Botvinnik used to do the same thing a lot of times... Of course all depends on your repertoire !!

It can be done in 5 moves, right?

Also maybe even more commonly... in the 150 attack and Bg5 pirc.

In the veresov white often castles very quickly on the queenside.

For example:

See, black makes moves that make white react with moves other than castling.

Players have to handle (at the moment) other important things, and then castle.

There's no rush if your opponent has no attack against you yet.

There's a line in the scandinavian if white plays 3. Nf3 with  5.. 0-0-0:

e4 d5 exd5 Qxd5 Nf3 Bg4 Be2 Nc6 d4 0-0-0

for white i cant think of any faster 0-0-0 than the Sicilian English attack as mentioned before me.

There are 3 entries in Chess.com's database for 1.d4 d5 2.Nc3 Nf6 3.Bg5 Nbd7 4.Qd3 c6 5.O-O-O, but it doesn't seem to give White an advantage or something.

9 0-0-0 in the Sicilian, English attack. Sometimes on move 8. Never got there on move 6 or 7.