Another update...
Was down in the basement and took a closer look at the boards that are nailed to the joists... they are nailed in with these nails:
Looked them up here:
http://www.uvm.edu/~histpres/203/nails.html
These ones are definitely hand-wrought... "rose petal" heads, tapered on all sides. I also have type A & B cut nails elsewhere in the house. I'll have to stop yanking them out though... the piano's bound to come crashing through.