Now there is a website called RunPee that seeks to eliminate (tee-hee) this problem by providing suggestions on when you should go during a particular movie. Theoretically, if you're about to go watch, say, Terminator Salvation, and you know you'll probably have to tinkle during it because you have a history of this sort of thing (you know who you are), you can check RunPee beforehand to see if other users have recommended a good time to go. And they have: For Terminator Salvation, you are advised to do your business approximately 50 minutes into the film, "when Marcus finds the pilot who ejected and helps cut her down." Below that is a description (scrambled, until you click a button) of what happens during the next three minutes, so you'll know what non-essential action you missed.
Since RunPee's content is user-provided, it lends itself to shenanigans like the recommendation for Hannah Montana: The Movie: "Go home right after the previews and pee for as long as you please in the comfort of your own home. You won't miss anything." (Har-har, but that joke's gonna get old fast.) It's also only useful if you remember to check before you go to the theater. Believe me, getting out your iPhone and checking the site during the movie is not advisable, as your fellow patrons may (and should) murder you. But if you can make RunPee work for you, more power to you. Soon RunPee will be the #1 #1 website!