One of the features in the latest July 2007 CTP version of GAX / GAT is the possibility to include switches in the installer execution at design time (that's the switches that will be added in the installutil execution when you register a GP through the Register GAT action, as show in the following picture...

Since some time ago, we're working in a new release of the T4 Editor . Maybe one of the most interesting feature is the ability to have intellisense in the contained language. Victor has posted some screenshots  that showcases that feature, check it out here.   Here you've a quick snapshot...