I struggled with this problem (or a very similar one) for MONTHS before I stumbled upon a simple solution that worked for me.
Disable Officescan realtime anti-virus
Not sure if it was the files that were being scanned in realtime, or if it was the firewall built into the realtime scanner that was causing the problem.
I did not have this problem before I upgraded to iTunes 8, even though I did have the anti-virus running before that.
In some cases, I could get iTunes to start without the "The Bonjour service has been disabled. Bonjour is required to share music with others. Please re-enable the service to use this feature." error, and I could connect to my remote speakers.
Usually, if I started iTunes first thing after a reboot, it would be fine. But then as I loaded other stuff, started my VPN (even stopped the VPN), and then exited iTunes and went back in, I would get the error, and then when I clicked OK, iTunes would start up, without the remote speakers.
So almost like a "latch". If I can get iTunes to start successfully first thing after a reboot, and I leave it open, then it works fine. But if I do other stuff first, then try and start iTunes, OR if I ever do other stuff and then exit iTunes and try and go back in, I get the dreaded Bonjour error listed above, and then the remote speakers are no longer visible in iTunes.
Not sure what the "other stuff" was that caused the Bonjour error when subsequently starting iTunes, but I believe it had to do with my VPN connection, which is just an ordinary windows XP network connection. Even if I stopped the VPN before starting iTunes, the damage had already been done, and I would not be able to start iTunes with remote speakers anymore until I did a reboot. (Note that my VPN connection is ordinary windows XP connection, not one of the 3rd party VPNs such as Juniper that is known to have problems with Bonjour.)
Then one day, my virus scanner was interfering with my web browsing (kept saying legitimate pages were virus risks - this has been happening more lately.) So I got annoyed and disabled the Officescan realtime anti-virus engine.
I then noticed shortly thereafter that iTunes would ALWAYS start up without the bonjour error, and I can ALWAYS see the remote speakers now!
So, must be some combination of something the VPN was doing (starting and stopping it, or just starting it), along with what the Officescan realtime anti-virus was doing.
My circumstances may be somewhat unique, and may not apply to others that are getting this bonjour error, but it just goes to show that turning off the virus scanner can fix this error, at least in SOME cases.
In the past, I was somewhat dismissive about people blaming the virus scanner for errors - seems like a "typical" excuse. But in my case, it solved the bonjour error! (I see that disabling virus scanner was recommended early on in this thread, and I WISH I had tried it sooner! I did try a bunch of other stuff recommended here, but not that.)
Going forward, I can play around with trying to get the virus scanner running again and seeing if there are some exceptions I can create for the scanning and for the firewall (BTW, my windows XP firewall is OFF, and I have a linksys router.)
At least I know what is causing the conflict now.