I only have a couple of minutes - I'll pm you some "depth" after I get through the Holiday madness.
The member who spoke so eloquently of mentoring your users is spot on. That is the relationship you want.
Google for the phases of change acceptance - like grief, there are several stages (though they escape me at the moment) and it helps to understand them (the phases) as you deal with your users. Remember that users will progress through the phases as different paces.
You mentioned a 50/50 buy in, and an AdminAsst who is your "champion." LEVERAGE their support and influences within your user groups. If you can - enlist them in a task group to help promote the new system's acceptance.
Your "supporters" should communicate with their peers and reports EXCLUSIVELY through the new system - even if it means they have to write an email, then call the recipient to say "check your email."
Have a brain storming session with the supporters (even if you have to do it one on one - or better yet - via email) to come up with more ideas like the previous one to get the users used to the new technology.
How big is the organization (user-wise)?
Try posters (yes, paper ones) in the break areas - that promote your cause, and remind people to use it.
Make it fun. Have an email based contest. Through a lunch or after work snack attack, and issue the invites via email.
Make it easy. Use posters and memos (yes, paper) to distribute tips & tricks. Increase the complexity of the tips as the users start catching on ... and gradually migrate to email based tips & tricks.
Since these are newbies, they may not all be comfortable with a mouse - be sure to distribute keyboard shortcuts.
Look for opportunities to show them how using the system can make their work simpler and easier - but don't be vague - be specific. Catch someone griping about waiting for something or using the sneakernet, and demonstrate how email would make that specific activity more efficient ....
Sally is walking back to her office bemoaning that she wasted the time running to Bill's office to see if he was ok with a report she gave him last week.
Walk back to Sally's desk, help her attach the report to an email, construct a subject & message to get Bill's approval, and send it. Then. Help Sally setup a reminder to follow up (electronically) until Bill responds - AND - a rule that alerts her to his incoming reply. Then stand there while Sally leaves Bill a voice mail to check his email for a critical communique.
While you are taking all this on - use the promotional ideas you get to help establish good email habits - reply vs reply to all, attachments, concise subject headers, , personal use, jokes, spam, viruses, etc., etc. You have an OPPORTUNITY to teach them "right," and the rarer OPPORTUNITY to not have to go in laying down the law and breaking bad habits - take advantage of it ;-)
Check out this post, and try to instill these behaviors as you are implementing the new system:
http://www.43folders.com/2005/09/19/writing-sensible-email-messages/
Also. Make sure that you understand any privacy and compliancy concerns your users may have - 1. so that you can implement safeguards in the system, and 2. Address those concerns directly as they may be an underlying cause of the reluctance.