Is it possible to code a page so that it's automatically added to the users favourites when they visit?

Thanks god, i's not. Unless you find some browser security hole handy.
The dialog box asking if the user actually wants to add the page will show up anyway.

The only way is to popup the browser dialog and allow the user to add it themselves. And this works (I believe) in IE only.

not worth it really, it was possible using a hole, a lot of users are using firefox now which isnt possible.

Also microsoft have released IE7 which i dont use so i doubt it.