The paper "Application of Two-Directional Time Series Models to Replace Missing Data" offers two methods (not necessarily under your precise model), one that minimizes "the average error associated with the missing value" (the other I can't understand from the abstract).
Edit. I have changed my (old) answer to community wiki. Would someone please vote this up so that the bot that reposts those questions for which there are no upvoted answers stops recycling this one? Thanks.