So... there is actually no official mechanism right now for getting the address which corresponds to the imported private key. Part of the reason is that you should generally already have that address (from wherever you generated/copied the private key) but perhaps that doesn't apply in your situation.
We'll look into adding something to help with this. In the meantime, if the private key is not already in the wallet, it will be added as the last key in the wallet. So after importprivkey you can retrieve the address using:
listaddresses * false 1