Hi Tim,
In the new beta release, you can grab a mapping dictionary using this:
(Dictionary<string, string>)timelineItemContainer.Metadata.Where(meta => meta.Key == GlobalMetadataKeys.TimelineItemUrlsDictionaryKey)
GlobalMetadataKeys is found in Seesmic.Sdp.Utils.
The dictionary that you get will have the proxy url as key and as value the original url (in case the url was not transformed, the value will be null/empty)
HTH,
George