If you feel the words 'male' or 'female' don't accurately describe your gender, Facebook is giving you options.

The Facebook profile is evolving. Facebook has already started toying with the idea of letting you be more anonymous, and now they're giving people who identify as transgender the opportunity to state the gender of their choice. When you go hit the Edit Profile link on your page and go to the Basic Information area, you can now decide to be something other than just 'male' or 'female.' Select 'custom' and you get a new set of options.

Facebook

I think you can expect this feature to get some abuse starting pretty quickly.