Estou usando o código padrão para uma função lambda:

console.log('Loading function');

var aws = require('aws-sdk');
var s3 = new aws.S3({ apiVersion: '2006-03-01' });

exports.handler = function(event, context) {
    //console.log('Received event:', JSON.stringify(event, null, 2));

    // Get the object from the event and show its content type
    var bucket = event.Records[0].s3.bucket.name;
    var key = decodeURIComponent(event.Records[0].s3.object.key.replace(/\+/g, ' '));
    var params = {
        Bucket: bucket,
        Key: key
    };

    s3.getObject(params, function(err, data) {
        if (err) {
            console.log(err);
            var message = "Error getting object " + key + " from bucket " + bucket +
                ". Make sure they exist and your bucket is in the same region as this function.";
            console.log(message);
            context.fail(message);
        } else {
            console.log('CONTENT TYPE:', data.ContentType);
            context.succeed(data.ContentType);
        }
    });
};

No entanto, recebo um erro de acesso negado:

2016-02-24T14:21:21.503Z    kvyo1midvc2r69gm    Loading function 
START RequestId: baf9049b-db01-11e5-bc34-791df91353a9 Version: $LATEST 
2016-02-24T14:21:22.500Z    baf9049b-db01-11e5-bc34-791df91353a9    { [AccessDenied: Access Denied] message: 'Access Denied', code: 'AccessDenied', region: null, time: Wed Feb 24 2016 14:21:22 GMT+0000 (UTC), requestId: '215CD9BB4094E209', extendedRequestId: '0kDBEyMiJYbMApEqJuAtKct2SKLI7Z7tCBVyW6QJsYwMHROvtCEDynbGSsBdqbwFcX+YrSlGnsg=', statusCode: 403, retryable: false, retryDelay: 30 } 
2016-02-24T14:21:22.539Z    baf9049b-db01-11e5-bc34-791df91353a9    Error getting object {"originalFilename":"c12eaadf3d3b46d9b5ded6c078534c11","versions":[{"Size":1024,"Crop":null,"Max":false,"Rotate":0}]} from bucket xmovo.originalimages.develop. Make sure they exist and your bucket is in the same region as this function. 
2016-02-24T14:21:22.539Z    baf9049b-db01-11e5-bc34-791df91353a9
{
    "errorMessage": "Error getting object {\"originalFilename\":\"c12eaadf3d3b46d9b5ded6c078534c11\",\"versions\":[{\"Size\":1024,\"Crop\":null,\"Max\":false,\"Rotate\":0}]} from bucket xmovo.originalimages.develop. Make sure they exist and your bucket is in the same region as this function."
}
END RequestId: baf9049b-db01-11e5-bc34-791df91353a9 
REPORT RequestId: baf9049b-db01-11e5-bc34-791df91353a9  Duration: 723.44 ms Billed Duration: 800 ms Memory Size: 128 MB Max Memory Used: ,34 MB 

Minha função lambda e meu bucket S3 estão na mesma região 'US Standart' e 'us-east-1', que são os mesmos

A permissão do IAM está ok para a função lambda, permitindo a ação GetObject (é definida com o assistente que cria a função lambda)

com toda essa verificação, não tenho idéia por que ainda estou recebendo o erro de acesso negado

desde já, obrigado