Estoy usando el código predeterminado para una función 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);
        }
    });
};

Sin embargo, obtengo un error de acceso denegado:

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 

Mi función lambda y mi bucket S3 están en la misma región 'US Standart' y 'us-east-1', que son las mismas

Los permisos de IAM están bien para la función lambda, lo que permite GetObject Action (se configura con el asistente que crea la función lambda)

con toda esa verificación, no tengo idea de por qué sigo recibiendo el error de acceso denegado

Gracias por adelantado