I'm currently playing a Priest(43)/Mage(43)/Warlock(43) group. I would say that having a Warlock/Hunter is a good idea, as you will have a tanking pet. I have tried groups with a Warrior, but running a real tank is a bit more complex then having a pet tank.

My priest is spec'd shadow and is usually in shadow form, but for boss battles it's nice to have the healing. Also, being able to bubble your AOEers is really nice.

I decided to go with 3 different classes so I would have some variety when "single-box" raiding with my guild.