Basically, there was a huge fallout between the company Mira (which hosted and promoted Mambo) and the core Mambo development team (independent, Open Source developers.) The development team decided to form their own development organization called OpenSourceMatters.org, and are moving to re-brand the code and continue development. Because Mambo was released under GPL, the development team has the legal capability to do this. Some call it a code fork, while others call it a re-branding.
Anyway, it has been officially renamed "Joomla" (which is the phonetic spelling of the Swahili word "Jumla", which means "all together" or "as a whole") and the team is currently soliciting sumbissions for an official logo and slogan. The team promises an upgrade path for current Mambo users.
Mambo will continue to be developed by Mira, so if you are a Mambo user, regardless of which route you choose (Mambo or Joomla) you won't be left behind.
By all means, go to joomla.org and have a look around, and make up your own mind as to which path you will follow...