This may be old news to some of you but it has proven to be so popular that I thought it was worth a mention here. This is a topic that came up on the forum a month or two back but I'll post it here to save you the trip. Basically someone asked how the PKG mechanism worked and Chris Lord, a Senior Program Manager for Microsoft Application Virtualization, posted this response:
If you'd like to read the thread in its entirety see http://forums.microsoft.com/TechNet/ShowPost.aspx?PostID=2880423&SiteID=17.
J.C. Hornbeck | Manageability Knowledge Engineer