Shared hosting environments usually run under a Medium Trust security setting. In Aspnet 2.0 this has changed somewhat, and one of the effects is that any strong named assembly must have the [assembly:AllowPartiallyTrustedCallers] attribute. This is a problem if you're using third party code and they haven't updated it.I will include this in PageMethod's next release. In the meantime, you can use the workaround with the configuration file.
One workaround is to set <trust level="full" /> in the web.config file (within system.web element) - although I suspect hosts shouldn't allow this to work.