Liquid Metal would most likely be cheaper & faster than milling given scalability of molding. Apple has the capital. I'm guessing against it for the upcoming refresh as we have yet to hear anything regarding material supplier & operations change.
This is too big of a change to keep secret. I would speculate they would roll out Liquid Metal in accessories first, then iPods/iPads, then iPhones, then MBP.
I'm sure the molding would eventually be cheaper, but just due to the fact that Apple would have to design and setup the manufacturing process would cost a lot of money. Sure, LM will eventually show up on Macs for a reasonable price, but just not until Apple works out the details. But, I could be wrong, and LM MBPs could be coming in the next couple of weeks