It is quite common in informal American English to use 'sure' as an intensifier -- to emphasize the truth of what is being said.
A sentence such as "It sure is cold today" is completely natural and common in everyday or informal AmE English. I imagine that it is used with the verb BE extremely often, and although it may tend to be used frequently with linking verbs, I don't think this sort of usage is limited to linking verbs. For example, I can easily imagine someone saying "He sure does know his stuff!"
Interestingly, I sense a difference in nuance between the following two sentences:
(1) "He surely knows his stuff."
(2) "He sure knows his stuff."
I might use sentence (1) in essence to contradict someone. In other words, if someone had said something that might suggest that he knows less than he should, I might use this sentence to say that I think it must be true that he knows his stuff since that is what I expect the case to be or that is what I think the case should be. However, it could also possibly be used to simply make an emphatic statement of fact.
On the other hand, I would use sentence (2) exclusively to state a simple but enthusiastic/emphatic observation of fact.
-By Amy