Dimensions of practice
These dimensions (and even more so this page) are work in progress - the dimensions are being published in a series of papers emerging in 2009.
Dimensions from the SPP
Other models and dimensions
Dave White has developed two dimensions which together provide a great tool for thinking about the differences between virtual worlds and other tools (including physical world ones I suspect though Dave doesn't go that far). Check out http://tallblog.conted.ox.ac.uk/index.php/2009/01/22/eventedness/