Here is a list of the top sites I've found when looking for information on movie screenings across the country.

43KIX http://www.43kix.com/mobile
Disney http://disney.go.com/index
Dreamworks http://www.dreamworks.com/
Entertainment Weekly http://www.ew.com/ew
FilmMetro.com http://www.filmmetro.com
Flicks and Popcorn http://www.flicksandpopcorn.com/
Focus Features http://www.filminfocus.com/focusfeatures/
Fox http://www.foxmovies.com/
Fox Searchlight http://www.foxsearchlight.com/insider/
Giant Magazine http://giant.blackplanet.com/
Gofobo http://www.gofobo.com
Grace Hill Media http://www.gracehillmedia.com/signup/
Hollywood Life http://www.hollywoodlife.net/
Lionsgate http://lionsgate.com
MetroMix http://www.metromix.com
MGM http://www.mgm.com/
Miramax http://miramax.com/screenings/
Movie-View http://www.movie-view.com
My Cinema Access http://www.mycinemaaccess.com/
MySpace http://www.myspace.com/blackcurtain
Paramount http://www.paramount.com/paramount.php
Premiere http://www.premiere.com/
Regent Releasing http://www.regentreleasing.com/
Sony http://www.sonypictures.com/
Sony Pictures Network http://www.sonypictures.com/spn/index.php
The Hollywood Reporter http://www.hollywoodreporter.com/hr/index.jsp
The Screening Exchange http://www.thescreeningexchange.com
The Weinstein Company http://weinsteinco.com/
Universal Studios http://signup.universalstudios.com/form/DEFAULT
Upscale Magazine http://www.upscalemagazine.com/portal/
Variety http://www.variety.com/
Warner Bros. http://www.warnerbros.com/