I like having composer support, but I really don't like making composer a hard-requirement.  It's a package/dependency manager, the autoloading stuff is a just a "side effect" as @ravage84 noted.

I suggest provide a simple fallback autoloader here? And have it run together with composer's autoloader if it is installed.
Eg: http://bin.cakephp.org/saved/108065

PR coming..