Hi,
In the Asset Store window, the available version of NGUI shows up as 2.5.1. However, when I import this version nothing actually changes for me (I'm currently on 2.3.1). I can verify this with my source control/Araxis, and even reading the ReadMe it says the version is 2.3.1.
I'm upgrading because since the Unity 4.1 upgrade, our checkboxes have stopped working properly - if they start checked they work fine, but if they start unchecked they can never be checked. This is solely a graphical issue, the functionality is maintained. The same issue is present in the Checkboxes Tutorial scene provided with NGUI, so I'm fairly sure it's a bug. One that I hope is fixed in a later version!