I would be happy to be able to use my warlock as a main tank. I was instantly amazed by the idea the minute these things came to the MoP-Beta.
I do want to emphasize though, that the ability to tank should come with using normal cloth caster gear or wearing other types of armor thats already available with tank stats. Nobody would be happy about further bloated loot tables only because locks need cloth with parry and dodge. In my opinion there should be a conversion of caster stats to defensive stats/abilities, so regular cloth gear could be used. Maybe spirit would be a good option for that, so tank-locks need to obtain at least another set of gear to be able to fit both roles, dps and tank.
However, I don't really see that coming.