Search warrants have to be very specific regarding what they target. "Give us all the records for those who searched for 'murder' related queries" isn't specific enough for a warrant. "Give us the records of [some user] searching for [terms related to some specific technique of disposing of bodies] during March 2009" might be valid, but IANAL.
If you're ever brought to trial using evidence gained by a warrant which didn't list that evidence to some reasonable specificity, any decent lawyer will have that evidence removed from consideration.
It would usually have to be even more specific than that.
i.e. they would probably have to specify what they were looking for as well as some specifics of why they think X user (IP or google username) relates to Y person in their investigation.
Were forgetting, I think, there is already wide provisions for law enforcement to get your email password.. no one ever gets het up about that in the same way but most of the big ISP's can be forced, with warrants, to hand over access to accounts :)
If you're ever brought to trial using evidence gained by a warrant which didn't list that evidence to some reasonable specificity, any decent lawyer will have that evidence removed from consideration.