I am not sure if this is intentional or not, but when creating and adding itemtype definitions, spaces seem to be automatically converted to "__x0020__" for the access string. This is fine, but when trying to retrieve the same definition using DgnEcInstance.GetPropertyValue, I must manually replace the space with "__x0020__"
ex:
IECPropertyValue property = appliedItem.GetPropertyValue(attribute.Name.Replace(" ", "__x0020__"));
Maybe this should be handled by the GetPropertyValue method in the future?