I usually ride within a few inches of the white line though, irrespective of the width - a practice I highly recommend, by the way - which gets me clear of most obstacles in the lane. It also puts me in a more visible position for cars turning from side streets, and should I need to leave the lane to turn right. Basically, I treat them the same as I treat any shoulder.
I do exactly the same. On my way to work, the first 5km is downhill in a 60kph zone, so I usually ride on the white line until I hit 60 then merge into the car lane to avoid having someone from a side street hitting me when they pull into the bike lane for looking room.
The bike lanes here are about 2.5/3 feet wide, debris is fairly light, most roads are swept weekly. The main reason for bike lanes is not so much for the cyclist but to help drivers judge the distance from the far side of their cars. The painted line on the far side gives them some sort of guideline to miss cyclists. I do find roads with bike lanes to be quite safe, I have never had any close calls with cars being too close. Adelaide is covered with bike lanes, at the moment there are some being put down right in front of my work.
CHEERS.
Mark