Hopefully, in the future Apple may add software support for voice recording, but in the meantime, this hint provides a basic way to record notes via voicemail. Dialing your number on your phone calls AT&T's voicemail, but automatically logs in to hear messages. As far as I know, there's no way to record a message in this mode. However, if you call AT&T's voicemail directly, you can enter your phone number and leave a message for yourself. With visual voicemail these messages won't clutter up your voice mailbox and work great as voice notes.
If you dial *#61#, the iPhone will display call forwarding information in the case of a call not answered (you can look here for more codes, or just google GSM code). The first phone number listed should be AT&T's voicemail number. Create a contact with this number, followed by a ',' (hold * for about 2 seconds), followed by your 10-digit cell number. When you call this contact, you'll hear AT&T's automated response, followed by your iPhone dialing your phone number, and then your voicemail greeting. Record your note and hang up. It may take a minute before you receive the message in your visual voicemail.
Mac OS X Hints
http://hints.macworld.com/article.php?story=20070803221625100