Art Traum (wow, almost typed in Thieme out of habit!) has mentioned that dadgad is an excellent tuning for people with limited mobility or dexterity but I don't know if he's actually done a lot of research. Interesting notion, huh? I remember that 3rd hand capo listing this as one it's virtues, on a related note.

Also, fyi, I've found that a proper setup (making sure strings seat properly in the nut) and a little pencil graphite for lubricant will help prevent strings from breaking when tuning frequently from standard to dadgad.

Bobert, I'm assuming you're counting the high E string as one which is why you think the D string tunes to G? From the thickest bass string to the thinnest high unwound string, it is D-A-D-G-A-D.

Good luck and have fun!

jim