Not sure why, the Mac Airbook SSD was reporting full. Tried to delete a bunch of apps, nothing seemed to help. I kind of wonder if it isn’t a time machine problem. I don’t have great OSX intuition and so an examination of the directory structure and process list didn’t really mean much to me.
Decided to just reinstall the OS. 40G free again. we’ll see what happens. Rennie pointed me towards The Omni Group – OmniDiskSweeper – Download or Tinkertool as decent utilities to use if it happens again.