Arnold Zwicky wrote today about
agreement with disjunctive subjects, i.e., clauses in which the subject is separated by an operator such as 'or' or 'nor'. He gives examples such as
Neither Barbara nor I {am, is, are} able to... and
If you or I {am, are} here.... This phenomenon is one that most if not all people struggle with, and I believe it provides solid evidence against the model of the brain as a computer which simply spits out answers based on some strictly written grammar code.
There are several competing motivations here. In the former example, one of these for many people is to say things that are prescriptively correct. This would be
Neither...is, because prescriptively, 'neither' is the "subject" of the sentence. Of course, most of us also want the verb to agree with the most immediate NP, in this case, 'I'. Yet
Neither Barbara nor I am... definitely sounds strange, because 'am' doesn't encompass Barbara. I think I would probably choose 'are', on the basis that this particular sentence is semantically equivalent to something like
We are unable to..., i.e., the subject is in some underlying way a 1st person plural. On the other hand, it is difficult to apply this kind of view to the second example. I wouldn't want to say 'am' for the same reason as above, but I also wouldn't want to say 'are' because of the conflict with the 1st person pronoun 'I', and the fact that because of the disjunct the subject is clearly singular.
I think that in some ways this can be explained by some resort to a person hierarchy in English. We typically want 1st person pronouns to come second, as in
Jim and I went to the store as opposed to
?I and Jim went to the store. But this hierarchy creates problems because of the above examples, where putting 'I' first would probably result in a more natural sentence (from a verb agreement standpoint).