Slothie on XDA-Developers has released a great utility to help keep track of your phone calls by automatically recording the audio of all your incoming and outgoing calls.
It automatically detects when you are on a call and then records the conversation to the folder you specify.
The software offers multiple codecs and bitrates for recording, and can record to your device or storage card.
At present the software in in early release, and only seems to work on English ROM devices, but the developer should fix this soon.
Read more about the software in this XDA-Developer thread here.