Nature and EMBO are together publishing "Molecular Systems Biology" and all basic research in this journal is Open Access. I am wondering why this has not gotten more press as it seems Nature is experimetning with OA models here. Nature has done some experimentation previously by making certian types of papers available freely (e.g., many genomics papers). But this is definitely one step beyond and they deserve massive kudos for it.
So if you are looking for a new OA journal to submit some systems biology related papers, you should try here. And maybe with a little effort, we can convince Nature it is worth doing for more of their journals.