Default_0_Bars.png (0-5) and FSO_0_Bars.png (also 0-5). They are located within a theme at /var/stash/Themes/<theme name>/Bundles/com.apple.springboard.
Default_UMTS_ON.png and FSO_UMTS_ON.png in the same folder. I don't know where the stock icons are, that is just where the replacements go. For EDGE, it's the same basic deal, same folder location: Default_EDGE_ON.png and FSO_EDGE_ON.png.