When using box-shadow: inset on an element with a border-radius applied, the inset shadow does not follow the radius. What happens: The shadow renders to the full width of the element, instead of following border-radius. What should happen: The inset shadow should cut off at the radius, or even curve down slightly with the radius (much like a photoshop bevel). This curve-down effect can be seen on a number of buttons around the internet, such as these: http://www.komodomedia.com/blog/2009/06/social-network-icon-pack/#icons The most technical way of describing it is a gradient is placed on the downward curve of the inset shadow. It is 100% opaque at the top, and 100%transparent at the bottom of the bevel.
Created attachment 48539 [details] Testcase for box-shadow bug
http://girliemac.com/sandbox/button.html This is another example of how this should render, with a live example as well.
This bug has been fixed in top of tree WebKit. If you continue to see the bug on a specific platform, please follow up with them.