3.5 multi-site in sub-dir

Can someone explain to me about this new option of being able to install multisite in a subdir? This sounds really good to me because, unless I misunderstand, this allows me to have my main website totally segmented and independent, say, a network implementation and hosting offering that I may wish to offer. Is this true? Is that the benefit of putting the multisite in the subdir?

Also, what about those of us who already have multisite set up? Will I be able to undo my set up and essentially move the multisite aspect to a subdirectory?

One more question... let's say my site xyz.com has multisite installed in a subdir, like: xyz.com/multisite... When my customers buy a site package in my network, will their site url be customersite.xyz.com like before or xyz.com/multisite/customersite ?