As I mentioned earlier this month, PlayStation Mobile would be getting a new SDK which had support for PSN leaderboards. Well it is out now and I have it downloaded and here are the official patch notes:
There are also some improvements to the way the Vita handles vertex buffers, which should bring a bump up in performance.
There are no titles that I know of that will have leaderboards just yet, but I will be sure to let you all know the moment I do.
[Source: PSM SDK 1.20.00]