Californians have been talking about splitting the state for as long as I can remember. But I think a lot of it must have been idle talk, because mostly it's faded away with the vague belief that somehow the feds would never let it happen. You two seem to have dug deeper into the question, and what you found agrees with what I know about the Constitution's mechanisms for admitting a new state. To work within the existing rules, one of the resulting states continues to exist with a boundary and perhaps name change, while the other state is defined as a new state and petitions Congress for admission to the Union.

Other than Puerto Rico, has a petition for statehood ever been denied?

A wikopedia articla http://en.wikipedia.org/wiki/Partition_and_secession_in_California says "...there have been more than 220 attempts to divide California into multiple states[1] including at least 27 serious proposals", and I believe it. Another one that's currently active would carve a new state out of a few northern counties and a few in southern Oregon. "Cascadia", I think is the name.

I'd vote for partition or even secession. The north and south have different characters and different needs, and administratively they're an awkward fit. A good sign that the split would be natural is the way that utilities have divided the state between them along the lines people use in partition discussions: PG&E in the north, Edison in the south. And once upon a time General Telephone owned the south and AT&T/Pacific Bell the north.

The present boundaries of California are as artificial and irrational as the boundaries of Iraq. Did the British design California too?