This bee is called "Ceratina smaragdula", a small carpenter bee that was introduced to Hawaii a few years ago. It's very close in size to the native bees, so it's potentially a serious competitor. It's widespread on the island of Oahu and was found on the Big Island in 2003. There were no records of this bee on Maui until I got these shots at Oluwalu in 2010.
I am now finding them along Kahakaloa coastline as well as along the south side of Maui.