The "long" and "short" used in English classes (not Linguistics) usually refers to the presence of a "silent e" and the lack of "silent e", but this terminology does not necessarily match up to a phonetic long and short vowel.
I remember hearing an argument in a linguistics class that for English oral stop consonants ("p","b","t","d","k" and "g" at the end of a syllable), the primary source of differentiation between minimal pairs, like "made" and "mate", is vowel length and that they are all devoiced.
I'm aware of the English-class terminology, but interestingly enough the guy bringing up "hit" and "heat" wasn't using that, as "heat" would be described under that system as having a long E, not a long I.
It's definitely plausible to me that (when the word is pronounced in isolation) word-final stop consonants are all devoiced. I haven't looked into the question at all and don't plan to, so I'm basically just spitballing. But my vocal tract can definitely start running down before I entirely finish speaking.
However, a much bigger issue than distinguishing "hid" from "hit" is distinguishing "hip" (where the final stop has become a glottal stop) from "hit" (ditto). I was under the impression that in general there isn't necessarily any phonetic difference at all there. If the argument you mention was only referring to minimal pairs with the same place of articulation, that sounds more reasonable.
I think final glottalization is a definite possibility, but I don't think it is a huge issue for native speakers given that context and word usage will resolve it for the most part.
I remember hearing an argument in a linguistics class that for English oral stop consonants ("p","b","t","d","k" and "g" at the end of a syllable), the primary source of differentiation between minimal pairs, like "made" and "mate", is vowel length and that they are all devoiced.
http://en.wikipedia.org/wiki/Vowel_length#Short_and_long_vow...