Angular http.post without .subscribe callback

I do not think you can.

http.post (and get, put, delete, etc) returns a cold Observable, i.e. an Observable for which:

its underlying producer is created and activated during subscription

Source.

This means the function represented by the Observable is activated only with the subscribe() method.

Convenience methods subscribe too, see implementation details for Observable#toPromise() here.

I had the same question but then I figured out that I actually don't care if someone subscribes to the observable. I just want the POST request sent in any case. This is what I came up with:

postItem(itemData) {
    var observable = this.http.post('/api/items', itemData)
        .map(response => response.json()) // in case you care about returned json       
        .publishReplay(); // would be .publish().replay() in RxJS < v5 I guess
    observable.connect();
    return observable;
}

The request is sent as soon as connect() is called. However, there is still an observable that the caller of postItem can subscribe to if required. Since publishReplay() is used instead of just publish(), subscribing is possible even after the POST request completed.

I'm using conversion to Promise (requires rxjs):

import 'rxjs/add/operator/toPromise';
@Injectable()
export class SomeService {
....
  post(sp: Seatplace, date?: Date) : Promise<any> {
     return this.http.post(
       '/list/items/update?itemId=' + itemId + "&done=" + done, 
        null
     ).toPromise();
  }
}