Yes, Joomla 1.0 will be essentially the same as Mambo 4.5.2.3. But there will be minor changes.
Joomla! exists, yes, inasmuch as the code is there and nearly ready to go. But since there has been no release of Joomla!, it's not "really" available.
Giving credit where credit is due is cool, but... The version of Mambo you are using was developped under the Mambo name, and you can't just go changing that willy nilly. The product you are using *is* Mambo, not Joomla.
As for the upgrade process, I'm not sure what steps will be necessary, but I'm sure the Core Team will make it as easy as possible
