String myURL = "<your site URL>/_vti_bin/owssvr.dll?Cmd=Display&List={List GUID}&XMLDATA=TRUE";
HttpWebRequest myRequest = (HttpWebRequest)WebRequest.Create(myURL);
myRequest.Credentials = CredentialCache.DefaultCredentials;
HttpWebResponse myResponse = (HttpWebResponse)myRequest.GetResponse();
Encoding enc = System.Text.Encoding.GetEncoding(0);
StreamReader reader = new StreamReader(myResponse.GetResponseStream(), enc);
string result = reader.ReadToEnd();
Source
You can chuck this URL into your browser as well and you'll get back your list data in XML. You can include the GUID of a view to filter what comes back:
http://<site url>/_vti_bin/owssvr.dll?Cmd=Display&List=<listid>&View=<view guid>&Query=*&XMLDATA=TRUE