Perforce is a version control system like CVS or SVN or whatever. So if EA has used Perforce for C&C3, they have been using it to hold all the game data (e.g. the source xml files etc) and not to actually do any compiling or anything else.
All the evidence I have (including debugging strings in the demo of the game) suggests that they have various source xml files (the names of which are visible in the compiled data) which are run through a tool (almost certainly EA specific and which may well connect directly to their Perforce repository to pull down the latest xml files and other data) which produces the .bin,.relo,.imp and .manifest files that we see in the C&C3 BIG files. (other files like textures and audio files are yanked into the build process too no doubt) What we will need is source XML files for C&C3 (including W3X files for 3d models). And then if we want to use the "compiled" form (which makes things load faster IIRC) we need the recompile tool (or rather, a version of it suitable for public use)
It may well be possible to do some deep level reverse engineering and figure out which XML tags and attributes are used for what and basically "reverse engineer" the XML files that way), I intend to play with the 3 uncompiled map.xml files from the 3 pre-order maps in fact. But I don't know how far such work can go and how much we can find out.
But really, in order to do any serious C&C3 modding we need to get more source XML files
