Lane splitting (or lane sharing for those with fears of negative connotations) is only legal in California and Utah. It doesn't stop most motorcyclists from doing it when necessary, but you have to pick your battles between getting ticketed or sideswiped or rear-ended. Haven't yet reached the same comfort levels with it as others. I like the idea of "threading" though. It sounds positive like sharing, but also productive like it is.
Admittedly these are all very abnormal circumstance. Each direction of the trip was almost doubled on the FDR and Westside highways, then the local streets were filled with additional holiday traffic trying to filter through and avoid the highway. Just happened to be the day I set aside for running errands... These are the times fault tolerances are designed for.