OK,
I'll answer my own question. I found the following info in an online manual for the Mitsubishi DP2070SB:
For the Mac: Connect the Diamond Pro Macintosh cable adapter (not in-
cluded) to the monitor connector on the Macintosh (Figure B.1). Attach the
15-pin mini D-SUB end of the signal cable to the Diamond Pro Macintosh
cable adapter on the computer (Figure B.1). Tighten all screws.
NOTE: To obtain the Diamond Pro 2070 SB Macintosh cable adapter, call NEC-Mitsubishi
Electronics Display of America, Inc. at (800) 632-4662.
I note that the G5 comes with a DVI to VGA adapter for VGA video out. I assume this also would have to part of the connection.