It worked!
Have you tried upgrading to a stable release of PHP? PHP is buggy at the best of times, but the release candidates are often badly broken.
I know it's working for you with other applications, but that doesn't mean it's MediaWiki's fault. There might be something different about the way we call the session handling functions, which is perfectly valid as documented but broken in 5.1.0RC1.
Thanks Tim! You're right!!! My wiki finally works, and it was due to the upgrading the PHP RC version the host was running. Now happily working away with version 5.2.0.
Thanks very much for tip. It was exactly what I needed. Took nearly forever to get the hosting co to do the upgrade, but they finally did this evening sometime.
I really appreciate the help here folks.
All the best, Scott