The PS3's operating system is very similar to Linux and as such doesn't become fragmented or need defragging. Only Windows stores data on a hard drive in such a half arsed way that de-fragging is required.
I've had my PS3 for over two years and the hard drive still accesses data at the same speed it always has.
Its down to the file system which windows uses (I think, I'm no expert tbh). NTFS which windows uses is quite prone for becoming fragmented. The PS3 uses a completely different file system (probably more similar to what Linux systems use) and which are less susceptible
Also the PS3 is used quite differently than a PC. There isn't as much moving files from one location to another, or installing and uninstalling different programs, hence less chance of files becoming defragmented.