To say that SAP ERP doesn't scale on the other hand is a bit nuts. It scales easily to the biggest companies in the world. Apple iTunes Store sales billing runs on SAP.
I've not spent a lot of time looking at open source ERP in the context of large enterprise. I'd be interested in a serious comparison of the contenders and their UX, ability to scale to many countries/currencies/languages and their ROI and supportability and extensibility.