One solution if you only need a list of string, is to declare a comma-separated (or any separator) list of values like this:

    [ConfigurationProperty("contacts")]
    [TypeConverter(typeof(StringSplitConverter))]
    public IEnumerable<string> Contacts
    {
        get
        {
            return (IEnumerable<string>)base["contacts"];
        }
    }

With this TypeConverter class:

public class StringSplitConverter : TypeConverter
{
    public override bool CanConvertFrom(ITypeDescriptorContext context, Type sourceType)
    {
        return sourceType == typeof(string) || base.CanConvertFrom(context, sourceType);
    }

    public override object ConvertFrom(ITypeDescriptorContext context, CultureInfo culture, object value)
    {
        return string.Format("{0}", value).Split(',');
    }
}

Your .config file would then simply be declared like this:

<configuration>
  ...
  <mySection contacts="bill,joe" />
  ...
</configuration>

Note this doesn't work for collections, when you always need to explicitely declare a property, like in Will's answer.