Ok, got a question. Pretty sure I already know the answer but it can't hurt to ask.

I have a High (comparitively) poly model all skinned and rigged. I have another version that I've cut down by about 80%. Is there a way to take the highpoly skinweights and stick them on the low poly version?

It would seem to go against what I understand about skinning, but given that's very little perhaps someone has some magic technique that I'm not aware of.

If you're using 3ds Max, you can apply the SkinWrap modifier to the low poly and go from there.

Yep, skinwrap has a convert to skin option. If you skinwrap your low poly mesh to your high poly mesh then click that button, it will pull the bone data from the high poly mesh (provided you used the skin modifier on the high) and give you a great start on the low poly skin.

Don't skinwrap the low poly to the bones that will probably make a mess because the bones are a drastically different shape then your low poly. If you're going to do that, you might as well just apply skin and start from scratch.

If the bones structure is the same you can also save out the envelopes (inside the skin modifier) and apply them to the low poly. But it would ignore the hand weighting or painting you did.

What about Maya? I should have mentioned that in the opening post but I was pretty sure I was just going to get a "You're plum outta luck, pally" typed response.

Use Copy Skin Weights.

Select all the joints that are weighted to your high-poly mesh, and bind them to the lower-poly one (regular Smooth Bind method).
Then select your high-poly mesh, followed by your lower-poly mesh (the order is important!) and do Skin -> Edit Smooth Skin -> Copy Skin Weights. Default settings should give decent results, but you can always try different settings to see which gives the best effect.

If you don't bind the mesh to the same joints initially you may have trouble because Copy Skin Weights assumes the joints are bound already.

If you by any chance have matching UV sets then you could also use the Export/Import Weights Map under Skin > Edit Smooth Skin >.