What's the most concise way to create a Task that never returns?

You can use:

await Task.Delay(Timeout.Infinite);

The docs states that the parameter represents:

The number of milliseconds to wait before completing the returned task, or -1 to wait indefinitely.

Timeout.Infinite is a constant field with a value of -1.

I hope I'm reading the question correctly here...

return new System.Threading.Tasks.TaskCompletionSource<object>().Task;

1. Return the task created by a Task Completion Source.
2. Never set the source to complete.

Continuations will never be invoked and await will 'block' forever.