I know this is an old, dead thread, but I'm sure people still find this page when searching for thermal pad info and I want to add my two cents worth. I still can not get over how many people do not know, or have doubts about the thermal conductivity of copper.
For anyone who does not know, or who doubts the thermal conductivity of copper, let me say that NO thermal pad will match the thermal conductivity of copper. Silver is the only "common" material that I know of with superior thermal conductivity to copper. And for those that think aluminum is such a great thermal conductor, aluminum has only about 1/2 the thermal conductivity of copper. If you don't believe me, look it up yourself. Unless thermal pads are imbued with some sort of non-existent thermal magic, they can not even come close to the thermal conductivity of copper. The copper is not the problem, it's all about proper fit and thickness and how well the surfaces meet, as well as using a high quality thermal paste on both sides. And of course, also being sure that the copper does not touch any electrical contacts and short something out.
Another thing to consider when using a copper shim is that many laptop heatsinks have multiple contact points (CPU, GPU, Chipset). On mine, the CPU makes direct contact with the heatsink and thermal pads are used on the GPU and chipset. If a copper shim was made too thick, this could possibly not allow the CPU to make proper contact with the heatsink and lead to even more serious problems. In the interest of simplicity, using a high quality thermal pad is often the best and easiest option. Also, keep in mind that all thermal pads are NOT created equal. A material with a higher W / mK number is better than one with a lower number.
Using an old copper penny to start with is not a terrible idea in a pinch, but it's definitely a McGuyver approach. However, not polishing the flat sides smooth is not really a good idea, especially if it will be used long term. I would also not use the penny whole unless I had no other alternative. It should be cut down to the size of the chip you will be using it on, and after it is cut to size, then polish both flat surfaces until as flat, smooth, and parallel as possible, and of course to the proper thickness. Once you think it is the right size, then use thermal paste on both sides and give it a test run. The downside of using thermal paste with a metal shim of any kind that is not fastened securely, is that over time and after thermal cycling, it can potentially begin to move. Once you know it is the right size and have tested that it will work properly and satisfactorily, it might be a good idea to use a thermal adhesive on ONE side of it to be sure it does not drift or move over time. Do Not use thermal adhesive on both sides or you'll likely never be able to remove your heatsink again without damaging something severely.
If you will be using your laptop in a warm climate, upgrading the stock thermal pads to higher quality thermal pads might be a good idea if you seem to be having heat related issues. This is one area manufacturers often skimp on, they use what is good enough in their opinion, not necessarilly what is best. FrozenCPU is a good place to get replacement thermal pads, and yes, they sell several different thicknesses, brands, and qualities of them. I hope this info helps someone, because I know the subject drove me crazy before I learned about it.