I found an easy way to rotate the screen from the Diamond forum. See
http://forum.ppcgeeks.com/showthread...=rotate+screen posted by TMZ where a zip file (called RotateScreen.zip) contains an executable file called RotateScreen.exe. As suggested in the post, I put this file in \Windows\Start Menu. RotateScreen will now be a selectable item in Settings-Menus. I added it there and now when I click Start, I can always select RotateScreen. After I start Garmin MobileXT, I just click on Start, then RotateScreen and it's in landscape mode.
Note that RotateScreen will work with other programs, but I was happy to find this for use with Garmin MobileXT.