Reactive Extensions: Throttle/Sample with varying interval

Many hours later, and with some sleep on it, I got it.

public static IObservable<T> Sample<T>(this IObservable<T> source, Func<T, TimeSpan> intervalSelector)
{
    return source.TimeInterval()
                 .Scan(Tuple.Create(TimeSpan.Zero, false, default(T)), (acc, v) =>
                 {
                     if(v.Interval >= acc.Item1)
                     {
                         return Tuple.Create(intervalSelector(v.Value), true, v.Value);
                     }
                     return Tuple.Create(acc.Item1 - v.Interval, false, v.Value);
                 })
                 .Where(t => t.Item2)
                 .Select(x => x.Item3);
}

This works as I want: each time it produces a value x, it stops producing values until intervalSelector(x) time passes.