Hi Brette,
The PartEditBox-derived controls don't have a IsReadOnly property like TextBox, but we have a TODO item to see about adding that property. The controls do have either an IsEditable or EditableParts property though. For example, DateTimeEditBox would allow you to prevent changes to the month part, while still allowing the days to be updated. For edit boxes with a single default part (like Int32EditBox), the IsEditable property plays the same role.
This doesn't work exactly like the TextBox IsReadOnly property though. It effectively disables the part, so it's text cannot be selected.