I actually bought a police style taser for my car since I can't pack a gun. It's the exact model the cops use (it looks like a gun and shoots out the darts).
I was never able to get an answer on the legality of it from the MSP. Individual cops said just be careful as to not pull it out near a cop (obviously since it looks like a gun).
There are specific areas where they do have laws specifically stating they are illegal. Baltimore and Howard county I know for sure.
http://www.taser.com/products/law/Pages/TASERM26.aspx
Anne Arundel County, the City of Baltimore and I think Ocean City ban Tasers even in your own home as well.. YOu can walk around with a loaded rifle, but you can't have the evil electric guns.
Except in those areas that ban it, the open carry of Tasers is legal in MD. It is the concealed carry that I have to look up again to see.