I wonder why you wanna switch between DNS-es?
I think there is no such app. Maybe there is if you jailbreak but I don't know
DNS Override can change DNS but via VPN connection
Well first of all I read on Internet that some DNS work well in a wifi/network but bad in another . But till now this is not my case .
I asked because maybe sometime a DNS provider has problem and so you switch to another
anyway , as far as I know , you need JB only if you want to change DNS for 3G , not wifi . In fact my iPhone is not JB