How to serialize a TimeSpan to XML

The way you've already posted is probably the cleanest. If you don't like the extra property, you could implement IXmlSerializable, but then you have to do everything, which largely defeats the point. I'd happily use the approach you've posted; it is (for example) efficient (no complex parsing etc), culture independent, unambiguous, and timestamp-type numbers are easily and commonly understood.

As an aside, I often add:

[Browsable(false), EditorBrowsable(EditorBrowsableState.Never)]

This just hides it in the UI and in referencing dlls, to avoid confusion.

This is only a slight modification on the approach suggested in the question, but this Microsoft Connect issue recommends using a property for serialization like this:

[XmlIgnore]
public TimeSpan TimeSinceLastEvent
{
    get { return m_TimeSinceLastEvent; }
    set { m_TimeSinceLastEvent = value; }
}

// XmlSerializer does not support TimeSpan, so use this property for 
// serialization instead.
[Browsable(false)]
[XmlElement(DataType="duration", ElementName="TimeSinceLastEvent")]
public string TimeSinceLastEventString
{
    get 
    { 
        return XmlConvert.ToString(TimeSinceLastEvent); 
    }
    set 
    { 
        TimeSinceLastEvent = string.IsNullOrEmpty(value) ?
            TimeSpan.Zero : XmlConvert.ToTimeSpan(value); 
    }
}

This would serialize a TimeSpan of 0:02:45 as:

<TimeSinceLastEvent>PT2M45S</TimeSinceLastEvent>

Alternatively, the DataContractSerializer supports TimeSpan.