This is an area where there are different opinions. Purists tend to go for the singular, but many speakers would use the plural. This depends very much on whether you take a prescriptive view (what you think people should say) or a descriptive (what they say) view.
Here are the Google results:
Is either- 27,600,000
Are either- 31,200,000
With figures like this, I feel it is impossible to say that one form is wrong, because it is hard to argue that such a number of speakers can be wrong, so I'd say both forms can be used, though I clearly tend towards a descriptive view of these issues.
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