ASP.Net MVC Controller Actions that return void

Yes

A controller that returns void will produce an EmptyResult.

Taken from

The Life And Times of an ASP.NET MVC Controller

Seems so, check the source code of ControllerActionInvoker.cs. I haven't verified it, but logic tells me that a void return will set actionReturnValue to null, so an EmptyResult is generated. This is the most recent source code, haven't checked the source for ASP.net MVC 1.0.

protected virtual ActionResult CreateActionResult(ControllerContext controllerContext, ActionDescriptor actionDescriptor, object actionReturnValue) {
    if (actionReturnValue == null) {
        return new EmptyResult();
    }

    ActionResult actionResult = (actionReturnValue as ActionResult) ??
        new ContentResult { Content = Convert.ToString(actionReturnValue, CultureInfo.InvariantCulture) };
    return actionResult;
}

It won't "convert" it, but the two would have the same effect as far as the user is concern. A request would be sent, but no response would come back to the client.

Personally, I think you need to send some response back to the client, even if you just write a continue or success directly to the response stream. Even a JSON true, or an empty XML document is better than nothing at all.