Google started blocking their new Movie rental service on all rooted Android devices. Google announced the service at Google I/O earlier this month, along with their music streaming service. The service allows users to rent movies to their android devices straight from the Android market. As it transpires, Google specifically locks out users of rooted devices for copy protection reasons.
You'll receive this "Error 49" message if you attempt to play a movie on a rooted device. Rooted devices are currently unsupported due to requirements related to copyright protection.
"Rooting" is a common practice in the Android world, this allows the removal of applications that came pre-installed which users may not want. Rooting allows the use of applications which wouldn't normally be possible on an Android device, such as screenshot taking apps and the ability to block ads and quite a bit more. It's also normally the first step before installing a custom ROM, such as the very popular CyanogenMOD.
There's a likely possibility that the movie studios are the ones behind the decision, unwilling to let Google license anything without some form of copy protection in place. Because of this, many legitimate users will feel they're being punished for the sake of preventing something from leaking on the Internet, which in all likelihood will already be there anyway.
Of course, the Android community will find a way around this.