A lot of people are now using Bingmaps (basemaps). These are in a different coordinate system than our local regional projection. The default datum transformation that the software uses is not the correct one; there is a shift of about 100 meters. The datum tranformation can be set in the data frame properties to correct the shift. The idea is to have the ability to set a different default datum transformation (similar to what you can do also for ArcPad). So when starting a new blank map document (normal.mxt) that uses these specific 2 coordinate systems, the correct datum transformation is taken.