You will probably find your meshes pivot point is out of line. Hit 'insert' on your keyboard. Then 'v' snap it to the a vertex running along the edge of your mesh. Then try the mirror operation again.
EDIT*
Looks like you have smooth mesh preview turned on. I would try switching it off and then performing the mirror operation.
Also, make sure all the verts are aligned and snapped to the x axis.
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Last edited by Mayaniac; 15-10-2009 at 01:46 PM.