Shocker, an 'actual' skinning thread 
There are two sets of frames, one , per pixel, supports transparency, dropshadows etc, the other older ones have to be magic pinked and do not support these things.
You need to skin both as some apps dont support per pixel and will default back to the older ones.
They also need to be cut differenty, Take diamond, the default skin.. open up all sections.. and jigsaw it together in PS.
That will give you some good default dimensions to work from.
Do this for both per pixel and non-per pixel frames and you can see how they vary in how they are put into SKS.... and you will have an idea of useable sizes.
Alternately, you can download this psd of mine that has some pretty default sized frames.
http://www.sendspace.com/file/v6s6ix
To make the older uis frames.. just turn off dropshadows and make the frames a solid color, then just merge the frames layers and "pink" around edges and background.
Look at an existing set of uis frames to see how they are sliced differenty. 
