It's now possible to drag a mapset folder to the Mac GRASS.app binary to have it open that mapset immediately on startup. This is exactly the same as the commandline startup (which doesn't directly work with GRASS.app):
grass63 /path/to/db/location/mapset
AND, the equivalent commandline startup for GRASS.app (so you don't need to type the full path to the grass.sh script or mess with your PATH) is:
open -a "GRASS-6.3.app" /path/to/db/location/mapset
Note that this will open a new Terminal window, instead of running it in the current window.
In working this out, I also seem to have fixed the odd PPC problem where running grass-xterm-wrapper once made it impossible to run GRASS.app again until Terminal is quit (which meant GRASS also must be quit).
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William Kyngesburye <kyngchaos*at*kyngchaos*dot*com>
http://www.kyngchaos.com/
"I ache, therefore I am. Or in my case - I am, therefore I ache."
- Marvin