>There are already privacy laws in place for medical information. As to the other, those issues already exist today. When someone's DNA is found at a crime scene, the majority of the time they are not related to the crime at all, but simply there for other reasons (visiting, family, business, et al). Those people are already ruled out during the process of the investigation, however, they try to collect DNA evidence after the crime now which makes it difficult to find some people (unless witnesses exist who saw you there at some time in the past) and rule them out. Also, DNA is used specifically to the crime. In other words, if your dna was on a disposed cigarette butt, but not anywhere near the victim (as opposed to on or near the victim), you would be ruled out after questioning (for anything you may have seen).
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>Perhaps John can correct me if I am wrong...
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Most agencies are collecting dna from prisoners, as they are processed. We also collect evidence at crime scenes, and for serious crimes, we photograph, take physical samples, etc. trying to preserve as much as possible for later analysis. Technology is helping more every day. We have an AFIS computer that solves a lot of crime just by pointing out the matching fingerprints.