In case anyone hasn't found them yet, Lion adds a whole load of new voices to the built-in speech (although the system has to download any new ones you select initially). Go to System Preferences->Speech, click on the System Voice drop-down menu and select Customize, once you've selected the ones you want to add they'll be downloaded via Software Update..
There are quite a lot of foreign-language voices, some better quality than others, and several regional English voices including English, Scottish, Irish, Australian and South African (no Welsh though, sorry Russ).
http://osxdaily.com/2011/05/15/mac-o...oices-samples/