Sharepoint's DocumentId.FindUrlById , used for http://kobikobi.wordpress.com/2010/09/29/sharepoint-2010-using-document-id-to-link-to-a-specific-version/

gistfile1.cs

//Sharepoint's DocumentId.FindUrlById , used for
// http://kobikobi.wordpress.com/2010/09/29/sharepoint-2010-using-document-id-to-link-to-a-specific-version/

public static string FindUrlById(SPSite site, string docId, string versionLabel)
{
    if (site == null)
    {
        throw new ArgumentNullException("site");
    }
    string[] documentUrlsById = DocIdLookup.DoSearch(site, docId);
    if ((documentUrlsById == null) || (documentUrlsById.Length == 0))
    {
        documentUrlsById = GetProvider(site).GetDocumentUrlsById(site, docId);
    }
    if ((documentUrlsById != null) && (documentUrlsById.Length == 1))
    {
        string requestUrl = documentUrlsById[0];
        using (SPSite site2 = new SPSite(requestUrl))
        {
            using (SPWeb web = site2.OpenWeb())
            {
                SPFile file = web.GetFile(requestUrl);
                if (file != null)
                {
                    SPListItem item = file.Item;
                    if (item != null)
                    {
                        SPListItemVersion versionFromLabel = item.Versions.GetVersionFromLabel(versionLabel);
                        if (versionFromLabel != null)
                        {
                            return (item.Web.Url + '/' + versionFromLabel.Url);
                        }
                    }
                }
            }
        }
    }
    return null;
}