[Edited December 9, 2011]
If you wish to repath the Lookup Tables for your office to a new location you'll need to consider that Revit 2012 has changed things a bit. The path may also be stored in a second Revit.ini file associated with your user profile (to better support user specific options).
The default installation should be here:
C:\ProgramData\Autodesk\RME 2012\Lookup Tables\Revit.ini
The user specific one is probably in a folder like this one:
%USERPROFILE%\AppData\Roaming\Autodesk\Revit\Autodesk Revit Architecture 2012\Revit.ini
If you find Revit is unresponsive to your new location, check the user folder location. You can read more about it in a Revit Clinic post from August 2011.
Original Post follows:
Pipe Fittings use a special folder to determine the correct fitting sizes when you create or change piping. The path may vary a bit depending on the operating system you are using. The folder, called Lookup Tables, is stored along with the content library, wherever that may be on your computer. This location is found in the Revit.ini file too.
If you move the folder elsewhere and change the location in the .ini file Revit doesn't acknowledge it. It still looks in the default location and now your fittings don't work. Seems to be hardwired...seems buggish to me.
Perhaps they didn't intend to give the impression that it could or should be moved to a central location? They don't even have a topic for lookup tables in the help documentation. Try a search for "Lookup" or "Lookup Tables"...no results.
The impression that it can be moved is given by the inclusion of the location in the Revit.ini file. Not the sort of thing your average "bear" will notice but those implementation minded "bears" will..guaranteed! In fact that's why I know about this...a fellow "bear" asked me about it! Now you do too!