A user recently asked about the steps needed to install the 2012 Revit Extensions on silent mode. Here is a suggestion:
1. Extract REX setup from exe to c:\autodesk
2. Go to c:\Autodesk\REX_RST_2012\x64\REX_RST\ (for x64)
3. Run CMD as admin, use command: REX_RST.msi /q
Especially important for Vista/W7 systems is to run CMD as admin, if not, nothing happens (no error message!).
Hope this helps.