Jim,
If you have loaded each DEM sepeately to the MXD then yes it will calculate the statistics for each DEM speately and then do a stretch. The toggle "From Current Display Extent" is only useful if the entire image actually fills the screen then it will calculate the stretch for that portion of image on the screen.
The work around to this is to include all your DEM's in a mosiac dataset (new Geodatabase format at V10.0). When you display the mosaic dataset the statistics are then calculated as if it is one seamless image (DEM) and the toggle "From Current Display Extent" will work for entire mosaic dataset rather than on the individual images.
HTH
Gordon Sumerling