What FW are you using on the PS3. I have a similar setup to Shonk; Twonky 4.4.18 on an LS Pro, and this works fine on our PS3 running the latest FW.
BTW, the clients.db entry I am using with 4.4.18 is:
So you can see that the PS3 now announces itself as PLAYSTATION and has dropped the 3 altogether. The "PLAYSTATION 3" / "PLAYSTATION3" problems stopped Twonky working at all when Sony changed to v2.40 of the PS3 FW so that isn't your problem as you can stream pictures and music.
I would look at your movie files - don't expect that just because a file works from USB that it will also stream. For that to work both ends need to support the file - the server and the player.