Thank you for sharing this beautiful site with us. I tried it on my own and I know what you mean. The current implementation uses a link to the java applet that opens in the same iframe and " history.go(-1)" to go back (which is the same as hitting the back button). The state of the google map gets lost, and with it the selected details.
A fix isn't simple or quick and not an issue of the iframe, it happens also when you use the inner link directly.
Simple (but ugly): Open links in a new window.
Complex (though nice): Instead of just using a simple link you create a postback which takes the current (Client Site State), persists it somewhere and redirects to your applet. The way back would be similar: Retrieving the state, load it to the client...
The complex task is not manageable for amateurs, you will need at least somebody with Google map development experience (best accompied with DNN knowledge).