The Relocate Masters option in Aperture 3 is very powerful and very useful. It is something I have been wanting in Lightroom for a while and unfortunately Lightroom 3 didn't deliver it.
When you import your images into Lightroom you can choose how they are stored, I use a preset to store them automatically in a date based hierarchy. You can move these folders around manually in Lightroom and you can also move them externally to Lightroom with the OS tools and then reconnect the referenced folder in Lightroom's catalog. But here is the issue, you cannot automatically re-arrange the files and folders after they have been imported.
For example: I imported all my files in a hierarchy of YEAR/MONTH/DAY but then decided I wanted to change it to just YEAR/MONTH. The only way to do this in Lightroom is to manually move several hundred folders or by re-importing and losing all my collections, virtual copies etc.
Enter Aperture and the Relocate Masters option: This can relocate any selection of images in any folders or hierarchy and automatically create and move them to a new folder destination. Its simple and fast and took about 10 minutes to relocate my YEAR/MONTH/DAY folder hierarchy to a YEAR/MONTH hierarchy with around 3,500 images. You would expect this sort of functionality to be standard in any photographic asset management tool but it is surprisingly lacking in many!