One of the most innovative features of the early Macintosh machines were that each file had a unique identifier. The file "name" was a user convenience, the software used the identifier.
Move a file? No problem. The OS knew where it was.
That's been more and more broken in the past few years. I don't know if the move is deliberate, or if it's a sign that Apple's most brilliant developers have all retired.
Alas, Time Machine demonstrates how broken this feature is.
...Time Machine doesn't handle the ol' switcheroo very well; renamed files and folders get backed up right alongside their previous iterations. This means that if you have a 2GB folder of images in your Desktop called "Purty Pi'tures" one day, then do nothing but rename the folder to "Rilly Purty Pi'tures" the next day, you just ate up 4GB of space on your Time Machine drive. Going further, Murphy Mac discovered that Time Machine makes some odd (and potentially unfortunate) decisions when throwing out backups. When Time Machine decides to retain a single backup out of the previous day's hourly backups, it chooses the first one of the day (presumably beginning at 12:01am). This means that some files could still be slipping through the sands of time (c'mon, I had to), depending on your computing habits...
It will be a sad day when Vista's file system is clearly better in every way than the Mac's.