I am trying to animate a model that I made using a mirror modifier. The mirror mod is still active when I add my armature using the methods in David Wards basic rigging lesson. When I did this yesterday it worked fine and both sides moved independently. Today after I parent the mesh and the skeleton, both sides move when I pose an arm. I have checked every setting for every object I can think of and see no reason this is happening. The only thing I did differently than yesterday was the x-mirror technique David showed, and even starting a whole new model this still happens.
I went and just tried it again on a fresh copy of Blender 2.5, and the only armatures I am putting in are the spine, and an arm just to check. The right arm still moves the mesh on the left side. I parented the arm bone to the spine and I’m using automatic weights to attack the mesh to the spine.
Please help me, I just want to get the rigging right so I can start animating this, I am close to using a fresh install of blender. Oh, blender 2.5 alpha BTW. Thank you.
have you tried applying the mirror modifier and adding vertex keys/ weight paint /heat paint yet?
I’m not really that advanced yet. So far all I’ve done is creature a central armature, and then create bones for the limbs. After I have them all set up I parent the mesh to the armature and do automatic weights.
ah, so you have asked this in the forum 
To get the armature to work with the mirror modifier the armature modifier must be below (after) the mirror modifier in the stack. There must also be empty vertex groups for the mirrored side (ie. if the left side exists with “.L” or “_L” suffixes, then create empty groups with “.R” or “_R” respectively. This should have been done automatically if the mesh was parented with automatic weights).
Symmetrical bone naming is very important here, so make sure you have named your bones correctly 