Property 'subscribe' does not exist on type 'Promise'

Following caffinatedmonkey's suggestion, I ended up with this working function:

    getPlanograms() {

    //API URL
    let requestURL = 'https://myapiurlhere';

    return Observable
        .fromPromise(this.storage.get('id_token'))
        .flatMap(token =>{
            let headers = new Headers({'Content-Type': 'application/json'});
            headers.append('Authorization', 'Bearer ' + token);
            let options = new RequestOptions({headers: headers});
            return this.http.get(requestURL, options)
                .map(response => <Planogram[]>response.json())
                .catch(this.handleError);
        }
    );
}

You could consume with .then() by changing:

ionViewDidLoad () {
    this.merchandisingDataService.getPlanograms()
        .then(Planogram => this.planograms = Planogram);
}

Or, you could make getPlanograms return an Observable.

getPlanograms() {   

    // API URL
    let requestURL = 'https://myapiurlhere';
    let headers = new Headers({'Content-Type': 'application/json'});

    // this converts from promise to observable
    return Observable.fromPromise(this.storage.ready()
        .then(() => this.storage.get('id_token'))
        .then((val) => {
            headers.append('Authorization', 'Bearer ' + this.authCredentials.token);
            let options = new RequestOptions({headers: headers});

            return this.http.post(requestURL, {}, options)

                // map converts from observable to promise
                // (returned by response.json())
                .map(response => <Planogram[]>response.json());
        });
    }));
}

Now you can consume with .subscribe() as you did in the question.