Ah, got it now!
Let's first define who's responsible for what:
The user (you) uses the controller to navigate the menu in the xmb. When a folder is being entered, the ps3 asks pms for its content and shows it. Thus, Sony has to offer a programatical interface on the ps3 where these commands can be sent through to use third party devices (iphone or other). Don't expect this to happen...
To mess things up a bit,
this post mentions another way to browse and watch films. It is being done through the web browser of the ps3. The guy even shows an ipad app to start/stop etc films. No release available though...