Here is a plugin for Maya 2009 (mll) and sources for Visual Studio (you can recompile the plugin for Maya 2011 or ask me to do it)
Workflow: 1. Load the plugin. 2. Two commands are now available ( saveWeights and loadWeight s) saveWeights skinCluster "fileName" ; loadWeights skinCluster "fileName" mirror? ; where skinCluster - skinCluster's name (skinCluster1, skinCluster2, etc) fileName - name of a file to save weight or for loading weights mirror? - if this parameter is set to 1 then weights will be loaded and mirrored according to X axis. Otherwise the weights will be loaded only.
Examples: saveWeights skinCluster1 "c:/weights.txt"; - save weights for skinCluster1 to c:/weights.txt
loadWeights skinCluster2 "c:/weights.txt"; - load the saved weights to skinCluster2.
loadWeights skinCluster1 "c:/weights.txt" 1; - load the saved weights to skinCluster1 and mirror them.