Heads next to the Ninth Circuit, which will doubtlessly uphold the ruling. Then it'll be on to the SCOTUS, which is a very different court from the '03 bench that ruled on Lawrence v. Texas.

Not necessarily. SCOTUS is very particular about the cases it submits cert for. The 9th cir. may stand, although it's liberal reputation won't help when it comes to the modern conservative court.

Why would the SCOTUS want to get involved? Are they in the business of deciding whether state supreme courts are correctly interpreting their state constitutions?

Actually, had you actually read the ruling, you would have noticed that the primary foundation for the ruling was not the California state constitution, but the Due Process and Equal Protection clauses [US Constitution, Amendment XIV § 1].

Probably soon, but still a step in the right direction.