is it just me.. or was it totally obvious that she was lip-synching that?!

I wasn't totally paying attention, but maybe there were some parts where she looked like she was lip syncing. But most of the time, I'm pretty sure she does sing the entire time.

I wouldn't put it past her sometimes though to lip-sync.

oh, i have seen her perform before and i was certain she was singing then! she does have an amazing voice with out of control range.. but im sorry, tonight really seemed fake to me.

Yeah I agree. She does have an amazing voice, & something did seem a tad bit 'different'.
I think Jordin Sparks was lip syncing to honestly when she performed on there last week.. Haha. So who knows, usually it's easy to tell, but I think you're right about Mariah.