An iPhone app like Speakeasy [iTunes link] is just what the doctor ordered. It lets you record messages of any length and then save them for later reference. Oddly, you can even turn a recording into a ringtone. It's $1.99.
Too pricey? iDicto [iTunes link] is just 99 cents. The app records messages like Speakeasy, but throws in some useful extras. Need to send someone a recording? E-mail it directly from the iPhone. The interface even flips upside down for easy access to the speaker.
Can't even swing 99 cents? iTalk Recorder [iTunes link] doesn't have many frills -- it pretty much just records, pauses, and saves -- but it doesn't cost a penny. You will have to tolerate some banner ads, though.