What on-line facilities are there for musicians to play together
across the internet in real-time, i.e. live bi-directional transfer of
MIDI and/or audio data in a peer-to-peer network?

yellowradio...

Due to the limits of internet connection speeds, I don't
think that what you're seeking has been done. Everything
I've read concerning collaborative efforts of musicians
via internet indicates that the pros use files which 
can be sent back and forth allowing tracks to be 
added by both parties in real time on their own PCs.

Real time jamming is still a dream, I believe.

You're probably right, in that efforts so far have involved
asynchronous transfer of audio clips that are then worked on
independently.  Rocket Network was probably the closest effort so far,
but that ceased trading in May 2003.  Given that Logic Audio had
support for Rocket, and that Emagic is now owned by Apple, Apple is in
a very good position to introduce its own subscription-based service
itself in the future.

With regard to your comment on internet connection speeds, though, I
don't think connection speed is that significant a problem any more
for broadband users.  The latencies can be very low indeed, depending
upon the network; I've had p2p delays of as little as 17ms (with a
Telewest 1Mb link, in the UK).  The next SF AES branch meeting
(http://www.aessf.org/meetings/Oct03.html) features audio over
Ethernet, by NetworkSound Inc (http://www.networksound.com/).

But if we consider streaming MIDI data as opposed to audio data, then
the proposition is completely viable.  MIDI uses very light payloads
(3 bytes for note on and note off at most), and the protocol itself
runs at 31250 baud, so I would assert that bandwidth is effectively
irrelevant, and that latency is the only concern for real-time
playing.

So, how about we coax Cakewalk into adding support to Sonar 4.0? :) 
We can piggy back off existing IM packages for the out of band comms,
and provided they avoid the temptation for a subscription charge, it
could really take off...

John

That's certainly a nice thought...  : )

I worked on a TV show where the hosts where constantly all over the
place.  This was not much of a hinderance, because there are good
quality sound studios with video playback in every major city.  I
didn't think much of it until I found out that the studios didn't
record the voiceover tracks, they merely fed it to another studio
locally that did the recording.  Don't ask me why they did it this
way, but the system was pretty advanced.  They could even patch in the
recording to different phone lines and conference calls so all the TV
execs in their offices could listen and give their two cents.
Additionally, I once saw a little documentary about the guy who does
the voiceovers for so many movie trailers.  he has a similar set up in
his home where he has a sound booth and a mic, but no recording gear.
all his tracks are patched to whatever studio or post-production
facility that actually does the recording.  I know this isn't a cheap
solution, and it doesn't address any latancy issues of two performers
trying to play in sync, but it would indicate to me that it can be
done.

Thanks, interesting comment.