For all the themers out there who didn't manage to make it to WWDC, here's an overview of some theme-related bits contained within the /System/Library/CoreServices directory in Tiger. Note that all of this may change before release.
- System icons are now in CoreTypes.bundle; SystemIcons.bundle is just an alias to CoreTypes now.
- Most of the widgets for the improved Finder search are in Finder.rsrc. the Localized.rsrc file inside the Finder package's English.lproj no longer even exists, though this may change before release (there's a button image inside Finder.rsrc with the word "Save" drawn directly on it in English -- clearly that'll have to change).
- The images for the blue Spotlight widget on the menu bar are stored in Search.bundle/Contents/Resources/MDSearchWidget_Normal.tiff and Search.bundle/Contents/Resources/MDSearchWidget_Pressed.tiff. The menu background patterns for the Spotlight menu and various other Spotlight-related widgetry is also stored in this folder.
There are probably more that I haven't rooted out yet, but most of the important ones seem to be accounted for.