GSA just did this same thing, launching Salesforce Chatter. Some of the important things that I could see (I was not on the implementation team) were:
1) Start small - pick a group or two to beta test so you can find any issues that may be unique to your organization before opening it up to the whole agency.
2) Have a team in place ready to answer all the questions that will come flooding in from the newbies AND have a FAQ ready and easy to find.
3) Set up a reverse mentor program where certain individuals are ready to help senior management get started with the product. If upper management is using the tool, and they see the value in it, so will everyone else.
4) Be sure to open it up to some non-work topics as well. I created a "Fitness Friends" group so people could share and talk about their workouts, find running partners, etc. Useful information, but it was also a great way to learn how to navigate the system, posting to groups vs. everyone, tagging names, asking for assistance, etc. - without potentially screwing up something work-related.
5) Share success stories. The IT team worked with the communications office on a series of feature stories highlighting how using collaboration tools like this can help your business/project/cross-channel communications. These features were sent out to the entire agency via email newsletter and promoted on our internal website to let everyone know what was happening.