Jeffrey Wang started the 2012 (or finished the 2011, depending on your time zone) by publishing the first article dedicated to DAX Query plan. While I look forward to reading next articles, it is interesting the explanation of what is the relationship between MDX and DAX in Analysis Services 2012.
An MDX query sent to a Tabular model is not converted in an equivalent DAX query, but at a certain point a DAX query plan is created (and probably many of them, both logical and physical plans). This is something that is very clear whenever you compare the DAX Query plan obtained from an MDX query and the one generated by an equivalent DAX query (You can find more examples in my article about converting queries from MDX to DAX).
Happy 2012 to everyone!