Actually, both Keyser and Shubb make leetle capos to cover just the 2nd, 3rd and 4th strings and get a "DADGAD up 2" tuning like Mr. Bridge said. Note that the "Tony Rice G" formation works normally for the subdominant chord, since you're fretting all the strings that aren't capoed.

The difference between partial capos and retuning is that any string you fret above the capo sounds just as if the capo weren't there. For instance, if you tune to drop D, you have to stretch all the way to the 5th fret for the bass in the G chord. With a "D-tuning" capo, the G chord form, actually an A, works perfectly normally.

I'm partial to using a "normal" capo at the first fret, and a D-tuning one at the third, and using D chords, when I need to play in F. Or in Bb - use G chords, but you get a nice alternating bass by just picking up the finger on the low E string, and putting it back.

Peter.