Return XML from a controller's action in as an ActionResult?

Use MVCContrib's XmlResult Action.

For reference here is their code:

public class XmlResult : ActionResult
{
    private object objectToSerialize;

    /// <summary>
    /// Initializes a new instance of the <see cref="XmlResult"/> class.
    /// </summary>
    /// <param name="objectToSerialize">The object to serialize to XML.</param>
    public XmlResult(object objectToSerialize)
    {
        this.objectToSerialize = objectToSerialize;
    }

    /// <summary>
    /// Gets the object to be serialized to XML.
    /// </summary>
    public object ObjectToSerialize
    {
        get { return this.objectToSerialize; }
    }

    /// <summary>
    /// Serialises the object that was passed into the constructor to XML and writes the corresponding XML to the result stream.
    /// </summary>
    /// <param name="context">The controller context for the current request.</param>
    public override void ExecuteResult(ControllerContext context)
    {
        if (this.objectToSerialize != null)
        {
            context.HttpContext.Response.Clear();
            var xs = new System.Xml.Serialization.XmlSerializer(this.objectToSerialize.GetType());
            context.HttpContext.Response.ContentType = "text/xml";
            xs.Serialize(context.HttpContext.Response.Output, this.objectToSerialize);
        }
    }
}

There is a XmlResult (and much more) in MVC Contrib. Take a look at http://www.codeplex.com/MVCContrib

use one of these methods

    public ContentResult GetXml()
    {
        string xmlString  = "your xml data";
        return Content(xmlString, "text/xml");
    }

or

    public string GetXml()
    {
        string xmlString = "your xml data";
        Response.ContentType = "text/xml";
        return xmlString;
    }