At the moment, Hasbro is preforming a webcast for their investors and has confirmed a few things according to SweetieBot who has been hanging out in the webcast. Apparently EqG WILL be in theaters and the Rainbow Kingdom playsets WILL tie in with the show somehow.
The webcast is wrapping up, but you can find it here. Interesting news I must say!
Update: Webcast is over and an executive said the following, that MLP will last five more years. If we assume one season per year we can expect at least five more seasons. To clarify this is only an assumption! As mentioned by others the current generation could last five years but not the show.
Expect a lot more pony in the future!