Adobe has released an update to Lightroom, 3.4.1. According to their website it includes:
Additional camera support for several new camera models including the Canon Rebel T3i, Nikon D5100 and Fuji FinePix X100
• Corrections for issues introduced in previous versions of Lightroom 3
• The Lightroom 3.4.1 update includes an important correction for a bug introduced in the Lightroom 3.4 release
If you have jpegs in your catalog, installing the update is very important, as the bug can corrupt a jpeg as you work on it/add a lot of metadata to it (it is rare, but why risk it).
If you open Lightroom and it doesn’t prompt you to update, go to Help>Check for Updates and download. Once it is downloaded, double-click on the folder to open it, and double-click on the installer program file to launch the process.
So that Lightroom prompts you when future updates come out, go to Edit (Lightroom on a Mac)>Preferences, and on the General tab, check the box “Automatically check for updates.”