It's reported that Washington lobbied for permission to release sensitive information regarding British missiles, but that they had refused. Britain has long maintained a policy of refusing to confirm the exact size and number of its nuclear arsenal. Allowing the identifiers to be shared would undermine this policy. The Telegraph stated:
The fact that the Americans used British nuclear secrets as a bargaining chip also sheds new light on the so-called “special relationship”, which is shown often to be a one-sided affair by US diplomatic communications obtained by the WikiLeaks website.
Details of the behind-the-scenes talks are contained in more than 1,400 US embassy cables published to date by the Telegraph, including almost 800 sent from the London Embassy, which are published online today
It's not clear where the American press is on this story. While all eyes are on Egypt, the White House may be able to keep this story off the front pages. President Obama has had a rocky first two years in his relationship with Great Britain. This could further erode the "special relationship" that exists between the two nations.
Update: There is a report stating the US State Department responds that this is simply a continuation from the earlier START agreement and does not constitute any change. There is some backing from the UK regarding the certain aspects of the US agreement to report information regarding missiles for 3rd parties. But the divulging of serial numbers appears to be a change and something the UK had apparently rejected.