Apple originally intended to address an apparent issue in connection with location based services that automatically check location without the user’s intention for them to do so and consequently lead to battery drain.
Apparently, the update did fix the battery problem but only for a small number of users.
However, since this is a beta release there are hopes that the final version of the software will be the one to fix the battery issue.
Did you update to iOS 5.1 beta? Are you still experiencing battery drain?