I found a couple of interesting articles on the blog of Troy Wilch over at myITforum which are related to the upcoming releases of Microsoft SCCM 2007 R2 and Application Virtualization 4.5.
The first post he talks about the different ways of delivering virtual applications, being streaming, local delivery and (virtual) MSI. In his second post he zooms in on these technologies and shows how the magic is happending behind the scene.
Then there's also a post about the scalability of the upcoming MAV 4.5 in larger environments. In this post he compares the "pre-softgrid 4.5 era" to features like Application Source Root (ASR), Icon Source Root (ISR), OSD Source Root (OSR) and the Overide URL which form the basis of the roaming functionality in version 4.5 on which SCCM 2007 R2 is relying.