It's important to remember that congress can pass no law (at least legally) that changes the meaning of The Constitution or one of its amendments.
I think Congress can expand rights, for example, as it did with the Civil Rights Act. But as far as the Third Party Doctrine goes, Congress didn't invent it. Its present incarnation Is rooted in a 1979 Supreme Court case: Smith v. Maryland: http://en.wikipedia.org/wiki/Smith_v._Maryland
Essentially what happened, is that Smith robbed a woman, she reported it to the police with a description of the robber and the car he was driving. The car was noticed, its license plate taken down, and the owner and his address ascertained from registration info. At that point, the cops went to the phone company and "asked" it set up a pen register. A pen register records the numbers a person dials but does not record the audio content, i.e., it records metadata. Smith called the woman he robbed and harassed her. Then he got arrested and the question is, should the phone evidence be tossed because the cops didn't get a warrant.
Obviously, Smith is an asshole and everyone wants to see him go to jail. But if the SC tossed the evidence, he'd probably get off. By the same token, the cops were assholes too -- they certainly had enough evidence to get a warrant to put on the Pen Register but they couldn't be arsed to do it.
Ultimately, the SC held that when you share information with a third party, like the phone numbers you dial (i.e, _share_ with Ma Bell), you have no expectation of privacy and the 4th Amendment simply doesn't apply at all.
This doctrine has been applied to records most people consider extremely private: The SC has ruled that it applies to phone and bank records. There is a split in the circuit courts of appeals on cell tower location information. There are even cases in some jurisdictions applying it to medical and pharmacy records -- your doctor and your pharmacist are in fact third parties. And no, HIPPA doesn't matter. The SC hasn't ruled on these latter examples so protection depends on where you live, but you're pretty screwed in Maryland:
In any event, the Third Party Doctrine is sort of like the Long John Silver standard -- the 4th Amendment only applies if you can maintain total secrecy, e.g., you have to kill anyone you do business with or lose your privacy rights.