The # symbol is commonly called a number sign, a pound sign (in the US), a sharp, and a hash among other things. It is not called a hashtag by anyone except people who think they are being clever and snarky. A hashtag is a tag prefixed with a hash.
The first time I saw this was in a Non Sequitur cartoon from May. Somebody tried to explain the confusion in the comments, but was ignored by the “Twittur yoosers r sooo stoopit” brigade. Since then I’ve seen it pop up on Facebook in other forms a few times.
Granted, a lot of Twitter users do seem somewhat deficient in the IQ area, but not because of the use of the term hashtag.
I think this distinction began getting lost when people started verbalizing hashtags. “Hashtag I’m a moron!” and so forth. That phrasing implies that the # symbol is indeed pronounced as “hashtag”.
Granted, I’m with you in wanting to stab in the eye anyone who does this, but that won’t help things, sadly.