In case you are too lazy to search to the Autodesk site... I've posted it here
Update 2 is not a full install; rather it is using service pack technology 
similar to AutoCAD®- based products. Update 1 is built into Update 2, so you don't need to worry about installing the first one before installing the second one.
Improvements made in Update 2 build (20110916_2132): 
Autodesk® Revit® MEP 2012 Enhancements
- Improves stability when routing ducts which utilize an elbow or takeoff to complete the run. 
- Improves stability when dragging a Panel Schedule into a sheet. 
- Corrects the classification of embedded windows within a curtain wall contained within a linked file which is used as the boundary to create spaces. 
- Improves the regeneration time of Ducts after applying a Visibility/Graphics Override.
- Improves stability when creating a new zone in the zone editor and specifying an already existing name in the properties palette. 
- Improves performance when adding an air terminal to a system. 
- Retains System Type when coping elements from one project to another. 
- Retains pipe and fitting connections when moving piping layout to another level. 
- Improves the display of duct with insulation and lining for hidden line views when contained within a linked file. 
- Improves display of insulation on segments when viewed in ceiling plans. 
- Maintains the type parameter values for panel schedules viewed on a sheet. 
- Maintains circuits when upgrading Revit MEP 2011 projects in Revit MEP 2012. 
Autodesk Revit® Platform 2012 Enhancements
- Improves stability when switching the visibility setting of linked file from By Linked View to By Host View. 
- Corrects Ambient Shadow on objects which are contained within a mirrored linked file. 
- Improves the import of IFC files which contain overlapping walls. 
- Improves stability when exporting a project to DXF which contains a dimension style that is referenced more than 255 times within the project. 
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Corrects the resolution of the background image when calculating offset and scale. 
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Corrects duplicate column creation when using Create columns by Grid when some columns are already created on the grid. 
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Improves stability when setting a view to Realistic display mode. 
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Improves stability when navigating in 3D view after canceling a Print operation.
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Corrects silhouette display for perspective views in projects with linked files. 
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Corrects the angle of Photometric Web based light sources when exported to FBX. 
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Retains the original IES files when exporting to FBX. 
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Reduces the scenarios where element IDs may become remapped or removed when Synchronizing to Central. 
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Improves stability when removing layers from floors, roofs and walls. 
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Corrects gbXML export of curtain walls where window openings were classified as air opening when using simple complexity mode. 
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Improves stability when accessing HELP
- Help
- Corrects mapping of Cable Tray, Cable Tray Fittings, Conduit and Conduit Fittings when exported to IFC. 
- Improves the consistency of beams being unique objects when exported to IFC. 
- Improves the placement of slabs when exported to IFC. 
- Improves the creation of beams, such as length, when imported from IFC. 
- Disables switching to 3D view if current view only is set when exporting to IFC. 
- Enables the export of filled regions to IFC. 
- Enables mass based curtain panels and mullions to be individual elements when exported to IFC. 
- Enables Structural Columns to be exported to IFC. 
- Allows elements hidden by category to be ignored when project is exported to IFC.
- Enables Unicode support for both Import and Export of families. 
- Changes setting for "Export views on sheets and links as external references" to be enabled when exporting to DGN. 
- Allows exporting to DWFx if the view is blank. 
- Improves stability when exporting to DWG. 
- Improves the export of custom wall hatch patterns to DWG. 
- Improves stability after displaying a warning message. 
- Enables the use of Family Parameter when using the Material Browser Paint tool. 
- Improves stability when modifying part assemblies. 
- Improves stability when working within the Phases dialog. 
- Improves placement of linked point clouds based upon their origin. 
- Improves stability when printing to PDF in batch mode. 
- Improves stability when saving a family into the same folder as the type catalog.
- Improves stability when upgrading projects. 
- Improves stability when setting a workset from Editable to non-Editable.