As a side note, if anyone is wondering how I test this then please download the modified medium trust file (called policy.config from Rackspace cloud) unzip it and save it to your web root folder.
Next simply add this to your web.config:-
<system.web>
<securitypolicy>
<trustlevel name="Custom" policyFile="policy.config" />
</securityPolicy>
<trust level="Custom" originUrl="" />
</system.web>
You now have a local development system running under medium trust. For those that do not run Rackspace cloud I would recommend asking your hosting provider for their medium trust policy file.This is an update from a previous post when using NHibernate 3.2 (September 2011). The JIRA issue can be found here.