I'm not up on the law with regard to weapons, but assuming there is a difference in charges regarding a gun and a loaded gun, the same argument could be made: you didn't have the right to search the contents of a gun? And yeah, that's stretching the comparison. =)
So how fine grained do you make the warrants? This is already established within the warrant system. As I understand it, you can't search a small locked safe if your warrant limits you to searching the property for a car. But in the case where a person is arrested for selling crack/cocaine, and during the arrest the person is searched, and the phone is found, how far is too far. After all, if a bag of white power had been found, would a separate warrant be needed to search the bag?
Anyways, it seems like this is something that should be addressed. While I'm in support of the 4th Amendment, it discusses unreasonable searches. And it seems reasonable to assume that if you get arrested for dealing drugs, it would be a reasonable thing to search the mobile phone you had for records.
Obviously, not a lawyer here. Completely arm chairing this. Corrections welcome.