TimeSpan.Duration() will give you the absolute value of a TimeSpan's duration which can be useful in some circumstances.
It is, however, a method rather than a property of the struct. The very first paragraph of the MSDN guidelines on choosing a method or a property states:
In general, methods represent actions and properties represent data. Properties are meant to be used like fields, meaning that properties should not be computationally complex or produce side effects. When it does not violate the following guidelines, consider using a property, rather than a method, because less experienced developers find properties easier to use.
Consider using a property if the member represents a logical attribute of the type.
(Emphasis theirs.)
Duration does not appear to violate the "use a method if" section, either.
I thought it might be for backward-compatibility reasons, so I went back to the property vs method guidelines for .NET 2, and they're relatively unchanged. So then I wondered if .NET 1.1 just didn't have properties, but it did, so I don't think that's the reason, either.