*WHOOSH* That's the sound of the importance of users' MAC addresses being publicized flying over my head. Why should anybody be concerned if the RIAA, or Google, or anybody else knows your MAC address?
I fail to see how your NIC's MAC address can be used to extract sensitive or private information. I don't know of any way that it can be cross-referenced or traced. Whenever you are requesting information from a server, doesn't every hop along the way replace the "source MAC address" in the IP packet headers with its own MAC address? The only way, then, for your MAC address to become relevant to outside investigations would be if your default gateway were examined.
Every investigation I have heard of involves IP addresses, not MAC addresses. Hypothetical situation: let's say EvilBob logs into his neighbor's unprotected wireless access point and torrents a few albums. Unfortunately, the RIAA was hosting those songs as bait to catch evil-doers like EvilBob who steal music. The RIAA decides to take action against EvilBob. However, they don't know EvilBob's name. All they know is his neighbor's IP address. They pull a few strings and probably convince EvilBob's neighbor's ISP to release account information such as the name and address under which the Internet service is registered. So the RIAA sends a letter to that address and EvilBob's neighbors scratch their head for awhile, then hopefully end up enabling some security on their access point.
Let's say EvilBob is a part of something more nefarious, like trying to incite riots against government officials. In tihs case, it's not the RIAA who goes to the ISP but the government. The ISP gives relevant information to the feds and they bust down the doors of EvilBob's neighbor's house and confiscate all their computer equipment. Soon they discover that there is nothing illegal on these machines and concur a neighbor was using the unprotected wireless network to perpetrate his crimes. At this point, the AP can be examined and the MAC address of a certain NIC (if it has not been spoofed) can be identified as belonging to the machine that sent/received said illegal material. If they want to find EvilBob, I believe they are giong to have to search the whole city block in search of that MAC address and hope that a) the MAC address they got from the AP was not spoofed and b) EvilBob wasn't parked outside and is 1000 miles away in another country right now, installing botnets in public libraries in Florida.
Why is anybody worried about this issue? I feel like I'm taking crazy pills here!