If you are writing an ASP.NET web application that is to be used in a hosted environment (like a forms application) set the trust mode on your dev box to Medium.
Many hosters will not let you run asp.net applications in Full Trust mode. So people end up developing applications that will not work when installed in a hosted environment.
There are many reason why hosters should NOT allow Full trust. If they do then I would fear for my own application.
http://msdn.microsoft.com/library/default.asp?url=/library/en-us/dnpag2/html/paght000020.asp provides more information on Medium trust in ASP.NET 2.0