The new .NET 4.0 changes the way that the page validation occurs. It puts in validation in more places now which can cause some issues.
<httpRuntime requestValidationMode="2.0" />
If you want to revert back to the old validation, just make the following change to your web.config.
Make sure you set the validateRequest = false on the page or in the Pages element.
You can check out what Microsoft has to say here.