In cases 1 and 2, as mentioned above by
rfc1394, I would
beg to differ. The proper
term in these cases would not be
unreadable, but rather
illegible, which is used when something is
impossible to read because of actual
physical conditions, such as
handwriting or the condition of the
material written upon.
I am unsure of whether this should also be applied to cases 5 and 6, but I think that the common usage leans more toward unreadable.