I have SIPForum.org openned in a tab, and this thought crossed my mind since I read a few posts that appeared on the Standards-JIG mailing list about VoIP and SIP's on my mind. If I recall the "proper" way was to translate or integrate with SIP. The thought that occured to me was this: SIP is trying to do IM extensions with SIMPLE and obviously doesn't care about working with XMPP, so why should we care about working with SIP, especially if it's going to hold our progress back trying to work with it?
There was also another post, quoted here, that mentioned a bunch of phone network features like caller ID and call waiting. Those two features in particular have no use on the XMPP network. I don't know about the others, but they may not be needed either. Something simpler (no pun intended) could be done.
At the very least, intially anyway, we can do a Jabber-Talkie. All I have to do is to select some contacts, select record, record a voice message, it gets sent to the receipients, their client receieves the audio file, notifies them that a [voice] message has arrived, and the message gets listened to. I imagine if this was done on a handheld it would be exactly like using a walky-talkie.
Granted that's not streaming and live, but it is in instant-messaging's vein, could be done extremely easy, and could be fun to use. And I'd rather have fun than have a baby-bell running on my server.
Push To Talk
The Jabber-Talkie thing is currently very hot and it's called 'push to talk' or PTT. A lot of (academic) research going on and some commercial networks are integrating it (IIRC ICQ has it now?).
It would be really nice to have this on Jabber, not sure if there are some efforts to get PTT on Jabber already.