SpyBug is a new Cydia application to secretly record voice notes with your iphone, it is activated by noise, when the noise level rises Spy Bug records the sound for the next minute, so I guess you can force a noise (like coughing) and the iPhone will start recording in silent mode, saving the recordings in its own folder.
You can download it by $ 0,99 on Cydia.
You need to have done the jailbreak.
Is this also valid for recording while talking on the phone? I remember my Nokia that I could do it and it was very good, for when you were talking and you had to remember or write down something like an order and you had no paper.