There are a couple of things I am not sure I agree with here. Firstly referring to this form of tablature as just tablature ignores keyboard tablature and harp tablature. Secondly, while I am aware of a few fifteenth century sources for tab, I am not sure about the legitimacy of saying that it was used in the Middle Ages. I'll do a bit of digging for that one though. Thanks for starting it though. This page is one of those things I have been meaning to get around to (along with more details on the troubadour page), but have never quite managed to. I'll upload a few images soon too.
Pardon me, I'm not sure yet how to work the 'talk' feature...
You're right, I did completely forget about keyboard and harp tablature. I'm too focused on my own predilections :-) And the New Grove agrees with you, that the earliest evidence is 14th Century so 'medieval' is probably going a bit far afield.