I recently sold an iPhone to my niece. There are some text messages between my wife and I she might find inappropriate. The phone is still on my account. Can I delete those messages before there is too much humiliation?
If you can get your hands on the iPhone you can obviously delete them the usual way.
If you won't get to it your only chance would be if you have setup MobileMe/Find My iPhone on the device. In that case you could initiate a remote wipe. This option however will wipe the entire phone including your nieces data.