We did one load of laundry for the first time on our last trip, Sept. 2006. We planned on doing it at the laundry building, while swimming in the pool. We stayed at POP. Unfortunately, it started raining as we were walking so the pools closed down. We did the load anyway. It costs (I think) $2 to wash and $2 to dry. The rain let up about 1/2 way through our load. So we just sat by the Hippy Dippy pool while by DD danced around, waiting for the pool to open.
As I said, this was the only time we've done it in about 10 trips. I do bring detergent, just in case, and I'll continue to do so. In the summer I usually pack enough(or too much) clothes anyway, since tank tops and shorts don't take much room and my DD7 clothes are so small I can get them all in 1 suitcase with room leftover. Of course, it would be ideal if you stayed somewhere with your own W&D. Then I would defininetly pack light. Also, less clothes to do when you get home. Now when we get home I just dump all the clothes, dirty or not, on the floor of the laundry room and start the loads.