I'd like to know as well except for whatever the generation is of Nano that is out there now. I don't want to lose the ability to charge the phone & the nano in the same dock.
I'm pretty sure you need a universal dock for that. A universal dock allows you to insert any dock connector. But I think you could still use the 3G dock for the nano it just won't fit nicely. I don't own a 3G iPhone dock so I'm not too sure.