During my time as a support engineer it was necessary to occasionally get netmon traces between Exchange servers, DC's GC's in order to troubleshoot exactly what was being sent down the wire - it wasn't always a pleasent experience.
Now, there is a new version Netmon 3 that will be RTM'ing fairly shorlty, meanwhile, if you want to join the public BETA you can do here
Here's what it can do:
Key features of Microsoft Network Monitor 3.0 include:
"There is a public beta available now for anyone that wants to sign up for it. Please point any of your customers to http://connect.microsoft.com and have them sign up for the “Network Monitor 3.0 beta” program. We already have over 2000 customers signed up since it was put on the web just over a week ago. There are also beta newsgroups, bug reporting, and a survey available."