Although I knew that the 1.1.1 firmware update introduced more attachment viewing support, I didn't realize until now that it added AMR playback. AMR, if you don't recall, refers to Adaptive Multi-rate Compression. It's the audio data compression format used by the iPhone for both voice memos and (if you've installed my Voice Notes application) voice recording. AMR is used primarily to store spoken audio.
So why is this big news? Well, if you're very very patient and you don't mind waiting for long gaps within a conversation, you can now use your iPhone as a very low-end push-to-talk device. By this I mean, you can install VoiceNotes and use it to record short messages. You can then email those short messages to a friend and they can play it back on their iPhone. To respond, they do the same: record and email.
This is obviously a long way from VOIP but as a stop-gap measure it works pretty well. I tested out a voice-only conversation the other evening and, while slow, the recording and playback quality was just fine for communicating.
Right now, the slowest part of the process involves addressing the message in Mobile Mail. If people show an interest, I'm considering either updating VNotes or writing an app that allows you to add the recipient's email once and then handles the addressing bit. Let me know in the comments.