When you are the dns root record and in many cases hosting too for some 60-70 million domains, someone in the internet is wanting to attack someone at godaddy all the time. There isn't a time they aren't being attacked somehow in all reality, and I'd presume it's much the same for Linode.
The means of really combating a ddos is costly and extensive, this is why most use a service like prolexic or silverline, and typically with some massive infrastructures that comes with it. Anything less than n-by 40gb disposable internet pipes, preferably regional as you are, you can/will be smited at will.
