Unfortunately I can't manually type the tags because the ESRI Mobile assemblies don't even show up as registered in my GAC.
Angel - Did you look in your GAC (C:\WINDOWS\assembly) to see if the ESRI.ArcGIS.Mobile assembly (or assemblies - maybe there should be more than 1, not sure) is there? It is not listed in mine, and I'm pretty sure this is related to the issue, if not the direct cause.