Does SQL Server evaluate functions once for every row?
Certain functions that are known to be runtime constants go through the process called constant folding. By 'folding' a constant an expression is evaluated early in the query execution, the result is cached and the cached result instead when needed. The expression in your query DATEADD(dd, 0, DATEDIFF(dd, 0, getdate())) is, afaik, a runtime constant and thus will be folded and evaluated only once per query. 

As trivia: the RAND() function that one would expect to be unfoldable is actually foldable, which leads to some unexpected behavior. But other, for instance NEWID(), are not foldable and will force an evaluation per row.