I think it was one of the firmware upgrades that made a change but it did it to mine as well. Basically you don't put it in MPblah-dee-blah anymore. It just needs to be in a directory called VIDEO. Are you signed up on the Playstation network? If you are you can go on there, download a free trailer and it'll create the directory structure for you and stick the video in it. The it's just a matter of overwriting it. Happened with me.
I'll check it out later on tonight but it's a ballache to get it all out and hook it up at the minute mate. I'll have a mooch and reply tonight. If you can though, do the trailer thing and you're in business.