Re: The 50th special will be feature length...
A movie release would need to introduce The Doctor to potentially new audience. I doubt the 50th would make a good movie, it would be too steeped in DW lore and history for your typical movie audience.
No one's saying it would be a movie, just that it would get a theatrical screening. Probably just something like a one night deal, like those TNG episodes that get theatrical screenings to tie in with the release of a new season on Blu-ray.
"Internet message boards aren't as funny today as they were ten years ago. I've stopped reading new posts." -The Simpsons 20th anniversary special.