There are by now alot of resources available for you.
There are some free open source applications. My understanding is that it may be more work than it is worth. You'll need to keep upgrading it for security as well as new widgets, mashups and applications.
Check out
37 Signals - they utilize the Ruby On Rails software. There are some other companies out there that use PHP, like
White Label Networking,
Pringo Networks,
Pilot Group's Social Platform,
Leverage Software, and a host of others. I orginially looked at the Dolphin software. It was $600, but in the end I really wasn't impressed with the quality of what it had at the time.
Marc Lesnick
Conference Organizer
Social Networking Conference