This looks like the same issue I am having, but since this is my first time using forge (slowly making the break from bukkit) I figured I'd introduce myself and post the error
EDIT:
ok this is the same issue as listed and/with a partial fix is here:
http://www.minecraftforge.net/forum/index.php/topic,10235.0.html
which suggests manual DL and copy /paste of this file
http://repo.maven.apache.org/maven2/org/scala-lang/scala-library/2.10.2/scala-library-2.10.2.jar
but you may need this file as well (I did) follow the same directions from the forum link i just posted above paste the .jar under .minecraft\libraries\org\scala-lang\scala-compiler\2.10.2 instead of "scala-library-2.10.2" and that should get you rolling
http://repo.maven.apache.org/maven2/org/scala-lang/scala-compiler/2.10.2/scala-compiler-2.10.2.jar
