[Snow]

I'm using this modified code:

        set nudge_dist 0.001
	bind .uvedit <Up> "ac3d uv_move_relative 0 $nudge_dist"
	bind .uvedit <Down> "ac3d uv_move_relative 0 -$nudge_dist"
	bind .uvedit <Left> "ac3d uv_move_relative -$nudge_dist 0"
	bind .uvedit <Right> "ac3d uv_move_relative $nudge_dist 0"
	
	set nudge_far [expr $nudge_dist*10]
	bind .uvedit <Shift-Up> "ac3d uv_move_relative 0 $nudge_far"
	bind .uvedit <Shift-Down> "ac3d uv_move_relative 0 -$nudge_far"
	bind .uvedit <Shift-Left> "ac3d uv_move_relative -$nudge_far 0"
	bind .uvedit <Shift-Right> "ac3d uv_move_relative $nudge_far 0"
	
	set nudge_fine [expr $nudge_dist/10]
	bind .uvedit <Control-Up> "ac3d uv_move_relative 0 $nudge_fine"
	bind .uvedit <Control-Down> "ac3d uv_move_relative 0 -$nudge_fine"
	bind .uvedit <Control-Left> "ac3d uv_move_relative -$nudge_fine 0"
	bind .uvedit <Control-Right> "ac3d uv_move_relative $nudge_fine 0"

As in Andy's code above, the value for "nudge_dist" can be changed.
Using the arrow-keys together with the Shift-key moves the selection 10 times as much as defined for "nudge_dist".
Using the arrow-keys together with the Ctrl-key moves the selection a 10th of the defined value.