Depends on which tanking spec you choose and how strict you want to stay to a tanking spec. The bottom line is DKs can still be pretty darn good tanks even in full DPS talent spec thanks to frost presence and 1 or 2 points you can put into oh-shit talents (like bone shield for unholy).

I'm guessing if you want to tank heroics, raids, etc, you'll need to forego some DPS talents but even still DKs still do WAY more damage then prot pallies, feral druids (in bear form), and prot warriors.