Arizona has some signs in Kilometers. [http://www.nytimes.com/2010/09/15/us/15highway.html?_r=0]
I suspect that metrication of the highway system in the US will only happen if a large state like California or Texas converts. But with the current budget situation, that's highly unlikely.
"the National Highway System Designation Act of 1995 prohibited the use of Federal-aid highway funds to convert existing signs or purchase new signs with metric units."
It appears from my reading of the law that the prohibition expired in 2000.
"However, Congress passed legislation in 1994 and 1995 that prohibited federal funding for converting highway signs, such as those for speed limits, to metric units."