Usually when a MS product finally goes to RTM, even if you’ve been playing with all CTPs and RCs, you’ll discover some ultimate (last-minute?) changes and new small features that wasn’t present before.
This one seems to be “just” a re-branding but may be significant of a new strategy for the (almost) new MS in-memory analytics engine technology : the Vertipaq engine which is now integrated in Excel through Power Pivot, Sharepoint through Power Pivot and PowerView, Analysis Services through Tabular Model and even in the Database Engine through ColumnStore Index (Codename Apollo) is now called xVelocity!
You can find the official announcement here on the Sql Server Team blog, and the usual strategy clarification here on the Analysis Services and Power Pivot Team Blog.
So … long live xVelocity !