2015 update: SAP has made available an Add-On to the SAP Business Suite for suppliers to connect to the Ariba Network, covering automation of sales order creation in SD from incoming POs, and invoice transfer from supplier to buyer via the Ariba Network. Check out the respective documentation at help.sap.com: http://help.sap.com/se4aribanet10sp07?current=erp-addons"SAP ERP Connectivity for Suppliers on Ariba Network".
For SAP buyers requiring integration with the Ariba Network for PO and Invoice automation or Discount Management, check out this SAP rapid-deployment solution: service.sap.com/public/rds-ariba.