Twice in the past week I have had the issue in my MOSS code where it is throwing an exception:

"The security validation for this page is invalid. Click Back in your Web browser, refresh the page, and try your operation again"

As it turns out the problem isn't so had to fix. Basically you need to set FormDigestSettings to disabled. This can be done in your code as follows:

SPWebApplication webApp = site.WebApplication;
bool formDigestSettingsEnabled = webApp.FormDigestSettings.Enabled;
webApp.FormDigestSettings.Enabled = false;
//Do all your code in here where it is throwing the error..
webApp.FormDigestSettings.Enabled = formDigestSettingsEnabled;

This can also be fixed by turning off the security validation for an application in Central Admin (however I probably wouldn't recommend this):

Central Admin -> Application Management -> General Settings -> Turn security validation off