Is your copy of music from the iTunes store the same as mine? I was wondering how the DRM/Purchased/Protected thing works with the songs downloaded from the iTunes store. 

If two people by a DRMed song from iTunes, are the raw bytes they download the same? Or are the songs encrypted using different keys per user such that they basically download different byte-streams?

I think they encrypt using a single key but I am not sure if that poses a threat because if even a single person can obtain the key by some hack, then it can easily be shared to everyone.
iTunes removed DRM from it's music catalog in 2009, so I'm wondering if your question is about iTunes at all? If not, you might be better off specifying what DRM scheme you have a question about.

That said, iTunes used FairPlay for DRM before, and as explained here, it does generate a unique key for every user. So, it will indeed be different for every customer. The same should be true for all DRM schemes, since if the customer could not be identified from the data inside the DRM scheme, it would defeat the purpose of DRM to start with.

It is also worth noting that while DRM was removed from iTunes, current downloads still contain your Apple ID in the downloaded file. However, tool exists to remove this meta-data from the media.