In Seoul, a metropolitan area of more than 20 million and with millions of cars on the road, lane splitting is the norm in all conditions. Most intersections will have between 2 and 10 bikes or scooters at the front of the traffic and it is normal to pass cars into on-coming lanes (when no cars are coming) or against the curb at the side of roads. My biggest worry is always the threat of punctures by riding along the curb to get by traffic (I've had 6 flats in the last year).
I've never ridden at home in the states other than doing a MSC last winter and I worry that if/when I return that I will be too impatient to ride there. The temptation may be too much for me to split lanes and I'm certain that the cops of Colorado would not be happy with me if I did.