Hi James,
Full Trust Access means that the user can call RevertToSelf() to end impersonation and run as the worker process identity. Generally all the
website hostings on a shared server run in the same worker process under the same identity which means the worker process identity will have read access to all web folders.
For security reasons we won't be able to offer Full Trust Access on a shared server as Full Trust permissions for ASP.NET applications will allow total control over the running process and may allow a malicious attacker to upload malicious code and execute it on the server.
Moreover, Web applications that are built on the .NET Framework version 1.0 always run with Full trust. It is said that in ASP.NET version 2.0 the Full Trust issue is more or less fixed and you can run those applications under Medium Trust.