Valeria Jones alleges in a lawsuit that co-workers repeatedly called Jones “miss,” “lady” and “little lady” despite explanations that Jones “was not a female or a male and that the term was unwelcome.” . . . . The lawsuit, filed this week in Multnomah County Circuit Court (Oregon), says Jones preferred to be addressed by a general neutral pronoun.The lawsuit apparently fails to include the employee's preferred terminology. We can speculate gender neutral terms like "their" (a plural that is catching on as a gender-neutral singular). This article mentions "zie" - a gender neutral term I have never heard of before.
Transgender issues are often tricky because many people are unfamiliar with the terminology and have little if any exposure to transgender individuals. I think a good general rule is to just call the employee whatever the employee wants to be called (unless it's a racial slur or something that would present its own liability issues).
As I have blogged previously, transgender/gender identity discrimination will most likely be treated as sex discrimination by courts.