I use Lightroom and not iPhoto, and I'm on a MacBook Air, so HD space is premium. I want all my PhotoStream photos to find their way into Lightroom. I've set up a Hazel rule that looks at ~/Pictures/iPhoto Library/Masters and searches all the subfolders for images and MOVES them into a separate folder that Lightroom then scans and processes, moving photos into its own folders based on date stamps.
It seems to be working well. All the photos in iPhoto are now broken links of course.
Any non-obvious downsides I should be aware of?