[X4U] move folder/file system callbacks?

[Xavier Noria]

Something really cool happened right now while editing an OmniOutliner 
document: I moved the file to a brand new folder behind the scenes 
while the document was open, something I had forget. When I switched to 
OmniOutliner and wanted to save the document, the program suggested to 
save it in its current correct folder! Magic!

I was pleasently surprised so I played around a bit and noted that if 
instead of the file itself I moved its folder, the application updates 
the file in its current location transparently.

First of all my admiration to the Omni developers for implementing such 
refined details. But I wonder how that can be accomplished technically. 
Can an application install some hook to be notified of files and 
folders moves?

-- fxn