I don't have anything to add about bamboo, but someone just sent me some samples of paulownia. It looks a lot like balsa and is quite low density (12 to 15 lbs/cu ft). Plantation grown, it can be harvested within 8 years. Said to have a good strength-to-weight ratio and not absorb resins too much. It seems harder than balsa, but lighter than cedar. The samples are a light cream color with some darker grain, but the grain is very widely spaced. They also sent me some "old growth" samples as well, still quite low density but more interesting grain and color. It looks like it has possibilities and probably not too expensive. Below is a link to the outfit that sent me the samples.

If people are looking for an alternative to cedar, it looks like paulownia might have some good properties, especially for those looking to make very light weight boats.