Is there an external bluetooth GPS antenna for Google maps on iPod touch WiFi?
Take a look at this http://xgps150.dualav.com/

My friend has this and he is using it with an app named Navigon (http://www.navigon.com/portal/int/produkte/navigationssoftware/mobile_navigator_iphone_eu.html)

I am not sure whether it works with inbuilt google maps, He is using Navigon for voice navigation and other useful features of Navigon. 

Hope this helps and answers your question