In Cali, when I was driving the big rigs, you didn't dare use the lanes unless you were really passing.
One of the fun games we get to play on I-5 through central California goes something like this...
On all freeways in California, the speed limit for big-rigs is 55, day and night. On I-5 through the Central Valley, the posted speed limit for cars is 70mph. Realistically, the riggers drive 65 or so, and the cars drive 80+, so there's generally a 15 mph difference.
This section of I-5 has two lanes in each direction, and as LDRyder said, the trucks will stay in the right lane except to pass. Here's the fun part.
When a truck decides to pass, he'll pull into the left lane at almost any time - quite often right in front of a car/motorcycle that's closing at 10-15mph faster than he's going. The truck will then drive about 1-2 mph faster than the truck he's passing, which causes a line of cars/motorcycles to back-up behind him in the left lane. It will take the truck a couple of minutes (literally) to complete the pass, so once he does pull back into the right lane, the cars who have been (im)patiently waiting behind him will speed up to make up for lost time. This dance continues for the 270+ miles between the base of the Grapevine and Stockton, in both directions.
You can't really blame the trucker - if he doesn't pull into an opening when he sees it, he could be stuck behind a (slightly) slower truck for 20+ miles. It does mean you have to keep on your toes as you overtake a line of trucks, 'cause you never really know when one will pull out in front of you...