In the 9.3 release of ArcMap, the Field Calculator is now ALWAYS disabled on the field context menu shown when you right click on a SHAPE (geometry type) field. This was always enabled in 9.2 and older versions of ArcMap but got taken away for some unknown reason.

 

But, you can still open the field calculator on SHAPE fields to do things like flip polylines or simplify, densify, and generalize features (lots of good .cal script examples for SHAPE fields can be found at: http://www.ian-ko.com/free/free_arcgis.htm#Edit).

 

The trick to opening the field calculator on a SHAPE field is to select the field (left click it in the attribute table) then hit CTRL + SHIFT + F to open the familiar window. This keyboard shortcut works on any editable field.

 

NOTE: For some reason the ArcToolBox-based Field Calculator does not seem to allow the SHAPE field to be utilized to access the geometry object for a feature. I have already run into problems because of this limitation.