'WaitFor' an observable

I'm in a situation where I have a list of Tasks that I'm working through (enable drive, change position, wait for stop, disable).

The 'wait for' monitors an IObservable<Status>, which I want to wait on (so I can thread it through ContinueWith and the other tasks).

I started out with the following tasks inside the OnNext handling of the subscriber, but that was just ugly. What I've now come up with is this extension method:

public static Task<T> WaitFor<T>(this IObservable<T> source, Func<T, bool> pred)
{
    var tcs = new TaskCompletionSource<T>();
    source
        .Where(pred)
        .DistinctUntilChanged()
        .Take(1)  //OnCompletes the observable, subscription will self-dispose
        .Subscribe(val => tcs.TrySetResult(val),
                    ex => tcs.TrySetException(ex),
                    () => tcs.TrySetCanceled());

    return tcs.Task;
}

(UPDATED with svick's suggestion of handling OnCompleted and OnError)

Questions:

• Is this good, bad, or ugly?
• Did I miss an existing extension which could have done this?
• Are the Where and DistinctUntilChanged in the right order? (I think they are)