Me da igual
 

I always thought it meant, either way is OK with me or either way is not OK with me, depending on the situation. Lately I have heard it to mean, what do I care if...

Is this a common usage?

It's both. :)

The canonical translation I would use for "Me da igual" would probably be "It's all the same to me," but it can also be translated as "I don't care" ("No me importa") or "It doesn't matter," ("No importa") etc.

I think the usage you described is common. Another translation of "What do I care if..." might be "¿Qué me importa si...?" but that's a lot stronger sounding than "me da igual."