I've heard of it referred to as a "hash" in computing since I was at college... which pre-dates Twitter a bit as I'm ancient.
I hadn't heard of the crossed lb thing before (and don't seem to have a compose key sequence to make one... I'll have to add it now in case I ever use it!) which is quite interesting. Randomly talking of octothorpe, I'm also reminded of the "Sputnik" symbol which is similar but with just single oblique points on its corners. I don't know what that's for either.
The pre-UTF UK code-pages also typically assign the same number to £ as # only with the high-bit set which I guess is appropriate if they're both pound symbols.
So I learnt something today. Now I'll probably forget something else to make room for it!
