According to a number of news accounts (like this one at Billboard.com), Rondor Music (a division of Universal), the publisher of the hit Beach Boys song, California Girls, has sent a demand to Katy Perry in connection with her current hit, California Gurls. Rondor is seeking compensation for an alleged copyright infringement. The gist of the claim stems not from the obviously similar title (titles of songs are generally not subject to copyright protection), but a similar lyric.
The main lyric of the Beach Boys' tune is "I wish they all could be California girls." At the end of the Katy Perry song, guest rapper Snoop Dogg improvises the line, "I really wish you all could be California girls." So, the question is whether this similar lyric legally entitles Rondor to compensation.
I could obviously write a whole article on the legal concepts underlying copyright infringement and how they apply in this case. However, this is an informational blog, not a legal journal, so let me make it short. The essential idea is whether the Snoop Dogg lyric creates enough of a legal similarity between the two works to trigger liability. (There might be other relevant factors, such as whether it was actually "copied" for legal purposes, but I think the similarity issue is the most pertinent.)
There are actually several different tests of similarity that courts have applied in these cases. Again without belaboring the issue, the most obvious fact is that this is one line out an entire song, and it's not even identical to the original line from the Beach Boys' song. On the other hand, it is the most prominent lyric from the original song.
Another consideration is that the idea behind a creation cannot be protected; only the expression of that idea. So, the question arises whether Snoop Dogg was only capturing the same idea, or actually copying the expression of the idea. And you could even argue that Snoop Dogg's rap is a parody of the original Beach Boys line; that would undermine Rondor's argument, as well.
All of these are good and legally relevant questions. Personally, I could make an argument for either side of the case. With that said, my opinion is that this does not rise to the level of an infringement. However, I don't think it will ever get to court. At this point, Rondor has not even filed an action, and I don't think they ever will. Some money is likely to change hands and the issue will quietly go away. Too bad really -- it would have made a very interesting lawsuit.