Tested on Maya 2009 & 2011, other versions could/should work too.....
// General usage :
Begin with selecting your groundplane on which you want meshes to be generated on. Press Prepare for Vertex Paint. Then, selected your meshes you want to be spawned on the groundplane and press Make system from Selection.
Vertex Paint on the groundplane with greyscale values and an alpha of 1 to actually see the painted colors.
If you're happy with your painted colors, press the bake texture button, set the emitter rate to for instance 10000 and check how it looks.
Further tweaks include random rotation, scale, align to surface normal, minimum distance between meshes.
You could put this under a hotkey for (almost) instant feedback while painting :
$exportMapSize=`floatFieldGrp -q -value1 $bakeGroup`;
$seedRate = `floatFieldGrp -q -value1 $seedGroup`;
$emitterRate = `floatFieldGrp -q -value1 $emitterRateGroup`;
Readme is included in the zip file with further instructions...
Other download link