The current internal version of SkinStudio Pro not only will let you create Windows Media Player Skins, WindowBlinds skins, Universal Skin Formatted skins but now can create icons of every known Windows icon format.
It can handle both individual and batch processing of images created in .PNG, .TGA, .BMP and other formats and automatically transform them into icons of various sizes.
I tried this out last night on a .PNG file I downloaded from the ObjectDock library last night and it worked great.
http://draginol.stardock.com/writings/skinni8.jpg (screenshot of final icon in various sizes).
SkinStudio comes in 3 forms: The free version which just creates WindowBlinds visual styles. The enhanced version which is $19.95 that adds widget, colorizing, and skin importing into skin creation. And the pro version which will have an introductory price of $29.95 and has Media Player skin support and icon creation. SkinStudio (all forms) is part of Object Desktop: http://www.objectdesktop.com
This new version should be up on ODNT next week.