As I understand it, SAP Netweaver is a repackaging and a reorganization of existing SAP Products plus some new stuff; repackaging means that R/3 and the New Dimension Products are tied together, reorganization means, that they will have all the same release level and scheduling.
As the different New Dimension Products support a different set of plattforms, I presume, that the platform supported by SAP Netweaver will be the lowest common denominator of all tied together products, and this lowest common denominator seams to me to be Microsoft Windows.
To force the issue, if for example a city administration decides to use only open source operating systems they cannot implmement SAP Netweaver.