Natural Widgets' NaturalRecorder is a Series 60 piece of software that allows you to record all your phone calls without any intervention on your part (besides installing and configuring the software). Previous cell conversation recorders suffered from memory management issues, but NaturalRecorder will fill your phone's memory up to the brim before it starts overwriting previous conversations. You can easily flag older conversations to be saved or select numbers not to record (your bookie, etc.).
Apparently the software emits a gentle beep every few seconds to let you (and your callers) know they're being taped, so it's use in blackmailing will be limited (although, "my battery is dying" might work for a minute or two). The software lets you play the recordings back right on your phone, and presumably, if your phone allow, one could also transfer them to your computer. $11.95, available now.
Update: Reader Adnans points out SpyCall, which appears to do the same thing, but is only $7.50.