From an interesting comment on this MacNN article: http://www.macnn.com/news/23527

"[...] the code on the cap is an access-control device to protected copyrighted iTunes songs... which means that telling people to look under the cap is a circumvention method, and surely Apple or Pepsi or someone can sue CNN for violating section 1201 of the DMCA.

[...] the soda itself is the key to the protection device, or at least it's opacity is. The soda water keeps the "secret music code" from being revealed in much the same way that the CSS encryption hides a DVD's contents."