Ohyeah, there are some scripts on highend3d for this, but I've never needed to use one. And wow, they really changed the site, I think i like it. (creativecrash, eh? when did this happen?)
the stupid thing about maya's "save skin weights" stuff is that it writes it to a texture based on UVs ... so if you have mirrored UVs you either end up with totally messed up weights, or you have to make a 2nd UV channel specifically for saving out the skin weights, then copy the UVs onto your "unskinned" mesh before…
You can go to the old file and export the skin weights. (should be under edit smooth skin menu) :) Then open the file you lost the skinning on and import them. You might be to re-skin first with just a default smooth bind or w/e. I'm not entirely sure. Check the maya documentation for the specifics. It saves the weighting…