Updating and expanding on this potential issue for Revit 2012 products; I have included the 4 most common causes for this behavior below, in the most common order:
1. WSCommCntr3.exe Process
For 2012 products, the Autodesk InfoCenter launches with Revit as a WSCommCntr3.exe process in the Windows Task Manager. The Revit.exe process could potentially wait on this WSCommCntr3.exe process. If you find Revit not getting past the splash screen, open the Windows Task Manager > Processes > WSCommCntr3.exe > End Process.
2. System Tray Applications
Verify if you have an application installed on the workstation, which allow the ability or attempt to minimize an application to the system tray. One example would be 4t Tray Minimizer. If present on the workstation I would test temporarily ending the application \ process to see if Revit 2012 launches as expected.
3. Corrupt WSCommCntrData.xml File
If the WSCommCntrData.xml file becomes corrupt, Revit may hang when attempting to read it at startup. First verify the file exists at the location below and if so move WSCommCntrData.xml when Revit is closed:
Windows Vista & Windows 7 ...\Users\%username%\AppData\Roaming\Autodesk\Revit\Autodesk Revit Architecture 2012\ENU\Components\WebServices\WSCommCntrData.xml
Windows XP ...\Documents and Settings\%username%\Application Data\Autodesk\Revit\Autodesk Revit Architecture 2012\ENU\Components\WebServices\WSCommCntrData.xml
4. Corrupt UIState.dat File
This could also be potentially related to a corrupt UIState.dat file. While Revit is closed, move or delete the UIState.dat file at the location below:
Windows Vista & Windows 7 ...\Users\%username%\AppData\Roaming\Autodesk\Revit\Autodesk Revit Architecture 2012\UIState.dat
Windows XP ...\Documents and Settings\%username%\Application Data\Autodesk\Revit\Autodesk Revit Architecture 2012\UIState.dat
Additional information on Revit 2012 file location changes: