from Dan Eicher (dna)
From the tracker (with minor edits)
========================
cube = bpy.data.objects['Cube']
foo = cube.vertex_groups.new('foo')
foo.add([1,3,5,7], 1.0, 'ADD')
for i in range(len(cube.data.vertices)):
try:
weight = foo.weight(i)
print('vert: %i weight: %f' % (i, weight))
except:
pass
foo.remove([1,3])
cube.vertex_groups.remove(foo)