Does anyone know if the Networking support in the new PLay engine will work across 2 computers using different platforms?(ex. Windows & Mac?) If this works it will save me buying some new gear and using VST system link in Cubase.
As far as I know it hasn't yet been implemented, (i.e. doesn't work) so I would be reluctant to make any decisions based on a promise. I would suggest that you check the Sounds_on_line forum for details of the current status.
Also, note the even when PLAY Networking support is implemented it will NOT transmit audio (I am not even sure it will transmit midi). According to the link above it definitely will include "the loading and unloading of patches and changing settings", but that is all it might do.
From reading I get the impression that PLAY networking support is similar in concept to remote screen control -- i.e. you can switch patches, etc. on the remote computer as if the software was residing on your host computer, but this does not include audio nor midi transmission.
Therefore, I think you still will need to handle the midi and audio connections on your own.