Two other solutions that are acceptable in some situations.
Modify the fuluentvalidation src code.
1.
Comment out the following lines in Src/CommonAssemblyInfo.cs
#if !SILVERLIGHT[assembly: AllowPartiallyTrustedCallers]#endif
OR
2. Add the following to Src/CommonAssemblyInfo.cs
[assembly: SecurityRules(SecurityRuleSet.Level1, SkipVerificationInFullTrust = true)]