Arnold Zwicky @ Language Log says:
(1) When all parts of a subject joined by or or nor are singular, the verb is singular; when all parts are plural, the verb is plural
…
The fact is that clause (1) (slightly amended, to get the person issue out of the way) is utterly uncontroversial; I’m not aware of variation on this point, and I doubt that anyone needs to be told what to do when confronted by disjunctive subjects of the same number (and person).
Am I the only person who speaks an idiolect on which:
- Neither Barbara nor Sam is able to …
is dubious and
- Neither Barbara nor Sam are able to …
is clearly correct?
I don’t need to be told what to do, he’s right about that. But I do the precise opposite of what he thinks everyone does.
nicolewyatt.net 
No, you are not. But, as I was remarking to Ann last week, more and more I am hearing people use “is” in the plural rather than “are”. I can’t tell if it’s a recent phenomenon or whether I am just now noticing it and it has been happenening all along. But it is somewhat disturbing.