THE FIRST BIG LIMITATION, you will only see these speeds when you stream a file type that the PS3 is compatible with.
The test, monitor your bandwidth on your PC media server! Open Task Manager on the PC running the media server. Select the Networking tab for your bandwidth graph. On the PS3 open up a 1080p .mp4 and press Square button. Move through the content as it seeks on the PS3 and watch your Task Manager on the PC. You will see gigabit speeds!
We all know (sadly) the PS3 is not Divx Plus HD cerified. This means the PS3 does not support the BEST and most efficient container format for ripping your Blurays! To play .mkv we need to transcode first using PS3 Media Server. Anything transcoded cannot take advantage of gigabit speeds! Instead we get errors when seeking with Square button.
Physical medium is dying. Don't you find it much more convenient to rip your media to a harddrive and play it from your couch? Yes we are a lazy nation! We are also losing jobs! Why not use a standard that saves the most space on our costly harddrives? SONY please support .mkv!
THE SECOND BIG LIMITATION, the PS3 throttles file copying speed to 75mb/s or 6MB/s! This is even limited on a standard 100mb/s connection. This has nothing to do with gigabit ethernet! I believe this is a limit imposed by software!
You can test this yourself! Monitor bandwidth on the PC media server using Task Manager on a standard 100mb/s network during a PS3 file copy. Notice network utilization is at 75%? Now perform a file copy over the network to another hardwired PC. You will see 98% utilization!