Hi Craig, the value type of a ChoiceField is an array. this is because a choicefield has the option to allow multiple or single selection. If you require a single value, the property should be saved as a 1 element array, which you should be able to retrieve using index 0.
You'll need to cast this to a string array first of course
<%# ((string)Eval("ChoiceField")) %>
also FYI, the team is aware of this issue and I'm told they are looking into ways to improve the choice field for many situations including this one. Once I know more I'll be sure to blog about it.
In the meantime, I hope this is helpful, thank you for your feedback!