The only parts of 'sharia law' of which I am aware have been supposedly been put into action in the US are: 1) Muslim taxi drivers refusing to serve inebriated passengers and 2) Muslim store workers refusing to handle pork/alcohol. I don't think either of these have been passed as law it has been only the individual's decision. I feel it would be against their human rights not to allow these people to practice their religion in this way. I feel sure that the constitution would stop sharia law been implemented if it even got close (which is unlikely). In Britain (where I come from) Muslims have set up sharia courts in which they can sentence people (just Muslims as far as I am aware) according to sharia law. In some cases this is probably not fair since the courts are said to be biased against women and the government has no say over what goes on in them but I don't think America (or Britain particularly) has anything to worry about.
The Constitution of the United States of America serves as the blueprint for the law of the land, not Sharia law as laid down in the Qu'ran and other Islamic texts. If Muslims are covertly implementing Sharia law in the U.S., then the Jews were responsible for the demise of Germany and caused World War II. The assertion that some covert Muslim movement to usurp civil law and order in the United States is afoot is absolutely ridiculous.
Several states in the past few years have enacted laws in their states that makes the practice of Sharia law illegal. Media will also give you the impression that there are cases being argued in court on the basis of Sharia law. However, if you really look at the fact, Sharia law could never be implemented in the United States because it goes again our current American Court System. You could not site Sharia law for justification of a crime.