I've just started using White and have a couple of comments:
1. Change UIItem.IsFocussed to UIItem.IsFocused (matches System.Windows.UIElement.IsFocused).
2. CustomUIItemType - Is there a reason this is used instead of ControlType? It looks like they're more or less one-to-one except that CustomUIType is missing a lot of entries. The White documentation advises you to create a new entry in CustomUIItemType
if you're creating a custom UI item and your type isn't defined. It seems like just using ControlType would save some pain.
3. Get<> for Lists - It would be nice to see UIItem.Get and UIItem.Get<> methods to return collections of items rather than single items, e.g. GetAll or something. This would be like using AutomationElement.FindAll instead of AutomationElement.Find.