At the moment this site is still on Mambo but when things quieten down I will be migrating across to Joomla! as I have a feeling it will have a stronger development path. I just wrote the following email to my friend in order to give him some information about Mambo/Joomla! so I figure why not post this up on the site so that if anyone else asks in the future the information in close at hand.
Joomla! is a completely open-source fork of Mambo. Mambo 4.5.2 code-wise is identical to Joomla! 1.0 except there's a few security/bug tweaks and different graphics are used. At the moment components/modules are compatible between the two systems but I can see a day when this won't be the case in the not too distant future.