The Red Alert 3 Mod SDK has (finally) been released. It enables players to mod the game to better suit their own or others' tastes, and includs a new graphical interface. For an introductory interview to the Mod SDK, read an interview with EALA SDK coder Justin Moe. You can get more details on the SDK here and download it from EALA here (130MB).
If you want to use the SDK for single player missions or anything else involving Worldbuilder, you'll need a custom version of Worldbuilder, downloadable here (600MB). For those modding the User Interface of Red Alert 3, use these UI reference files.