I would have to say that it is beyond a question of doubt in my mind that there will be more than 3 movies of DPPt. Yes, Sinnoh does seem to be wrapping up, but we have at this point had no mention of Gen V. Something else is going to happen with the anime before the start of the 5th gen, be it johto, battle frontier, or elite four arcs. There is absolutely no way that Ash, and whoever he travels with for the 5th gen will be happening come July 2010. Movie 12 will be happening right around the tail end of Sinnoh, but movie 13 will undoubtedly happen during some kind of filler arc.
Think about it, if they were to somehow manage to start the gen V anime by summer '10, it would have to be most likely at the very beginning. I would have to say that the best bet is that Ash will do the same thing he always does and leave his pokemon with Oak and get some new female companion. The movie would then be taking place when the characters had about 3 pokemon each, hardly useful when quelling legendary pokemon or what have you.
The pokemon probably won't be from the 4th generation simply because we're running out of 4th gen legendaries. The only one i could see it being is Heatran if the filler arc ends up being about a battle frontier located on the same island as Stark Mountain. Every generation thus far, bar the first, has had at least one movie before its actual release with a pokemon from that generation.
About the 12th movie being the last for DPPt, that's a lie. It will, I'm sure, be the last movie to take place in Sinnoh and therefore be the last movie about the main Sinnoh legendaries. Movies 10, 11, and 12 will make up a Sinnoh trilogy, but it won't be the only movies for DPPt. Movies 8 and 9 were both AG while starring gen IV pokemon, so the same will very well happen with DPPt.