A bubble is simply a deviation from the historic trend line. It's the formation of a bubble shape in what should otherwise be a straight line.
Whether or not we've begun a new bubble depends on whether you think we've overcorrected. My sense is that Irvine never did overcorrect and may not have corrected enough. In other words, it never retouched the historical trend line like the rest of OC did.
For the majority of OC, prices did overcorrect between 2009-2011 and the price increases occurring now are bringing things back in line with the historical trend line, meaning it's not yet the beginning of a new bubble.
For Irvine, this seems to be the continuance of the previous bubble because prices are back to 2005 levels in some neighborhoods, and you also have the IRRATIONAL exuberance necessary for a bubble to occur. Many residents believe Irvine is immune and FCB's are throwing so much money around that for now it is immune.
This is an interesting question though.