I'm not sure if this is a bug from the Workspace part or not, but I was trying to start a new process instance on my newly deployed process which has an XmlField["Document"] in it. I have not try this using the real thing (ASP.NET creating ProcessInstance, etc.).
I inserted some xml content that has a DateTime formatted as such "2008-04-22T14:32:40.940403+07:00", when I read it back from the ProcessInstance.XmlFields["Document"].Value and try to parse it, it blew up. After doing some debugging, it turned out that my DateTime field has been changed to "2008-04-22T14:32:40.940403 07:00". Note the missing + sign (for the timezone). Hmm...