I used this little snippet after a couple of hours hunting MSDN forums
StreamResourceInfo f = Application.GetRemoteStream(new Uri("pack://siteoforigin:,,,/"+fileSolutionPath));
//f.Stream //to your IO with this
Note: siteoforigin files are downloaded from the host on demand, so should be downloaded as needed.
Cool little beasty
http://msdn.microsoft.com/en-us/library/aa970494.aspx#Site_of_Origin_Files
http://social.msdn.microsoft.com/Forums/en-US/wpf/thread/9df060ca-019e-4443-8167-ccf511a73f84
I actually used this to copy some configuration files, DICOM Server Association files, to my Full Trusted XMAP home. I had made the Full Trust XMAP home (Environment.CurrentDirectory) a Temp folder i.e. System.IO.Path.Combine(System.IO.Path.GetTempPath(), "MyAppName") and needed to get the association list for a Storage SCP.
Thus
f.Stream.CopyTo((File.OpenWrite(Path.Combine(Environment.CurrentDirectory, fileSolutionPath))));