The New York City Police Department has unofficially made public their intention to beta-test Google Glass for use in the field. In a chat with Venture Beat, what they call a "ranking New York City law enforcement official" has spoken up on the subject, apparently wanting not to be named specifically. At the moment it would appear that they're only looking at the headsets, "seeing how they work."
The group would have had to go through Google's "Glass Explorer" program or would have had to have gotten invites from Explorers that've already purchased Glass units. It's also possible - though unverified - that the NYPD spoke directly with Google to attain the right to purchase the headsets.
"We signed up, got a few pairs of the Google glasses, and we’re trying them out, seeing if they have any value in investigations, mostly for patrol purposes.
We think it could help impact patrol operations in New York City. We shall see." - ranking New York City law enforcement official
Meanwhile Google appears to have suggested directly that they are not working with any law enforcement agencies. Instead the extended Glass Explorer program seems more likely for the NYPD's supposed route.
Google Glass has recently been a part of the Super Bowl as well as a Sacramento Kings basketball game, but thus far there've been no direct connections between the headset and law enforcement in the public as such. Unless you count the dismissed citation for a Google Glass wearer in California, of course.