You can use ConfigurationConverterBase to make a custom configuration converter, see http://msdn.microsoft.com/en-us/library/system.configuration.configurationconverterbase.aspx
this will do the job:
 public class CaseInsensitiveEnumConfigConverter<T> : ConfigurationConverterBase
    {
        public override object ConvertFrom(
        ITypeDescriptorContext ctx, CultureInfo ci, object data)
        {
            return Enum.Parse(typeof(T), (string)data, true);
        }
    }
and then on your property:
[ConfigurationProperty("unit", DefaultValue = MeasurementUnits.Pixel)]
[TypeConverter(typeof(CaseInsensitiveEnumConfigConverter<MeasurementUnits>))]
public MeasurementUnits Unit { get { return (MeasurementUnits)this["unit"]; } }
public enum MeasurementUnits
{
        Pixel,
        Inches,
        Points,
        MM,
}