I just published an article about Sorting Dates Columns in PowerPivot v2 and BISM Tabular on SQLBI website. In reality, the article is about sorting columns by other columns in a Tabular model, but the common scenario in which you need that is in a Calendar table where you can have columns that have to be sorted by using a combination of other columns and not just once.
For example, a MonthYear column containing January 2012 has to be sorted by Year and then by Month. In a Multidimensional model this is usually done by defining a composite attribute key for such attribute (which will be defined by Year and MonthNumber source columns, exactly in this order) and by selecting the AttributeKey value for the OrderBy property of MonthYear attribute in the Date dimension.
The lack of attribute keys in a Tabular model has several consequences that include this one. Other issue is the inability to save an item key for a report different than the visual text (which is an issue if one day an item will change its description but not this key). Another related issue is related to translations: today Tabular doesn’t support translations and the lack of an item key is not a real problem for this specific scenario, but I’d like to know how Microsoft will handle Translations one day without introducing a concept similar to Attribute Key that we have been using in Multidimensional since the first version of Analysis Services.