Vista ultimate is a the grand daddy of them all. It includes the Vista Home premium multimedia stuff and the Vista Business networking like accessing a domain. There are some additional features in Ultimate like bit locker for encrypting your hard drive but that can kill performance.
I am not sure what the advantage would be to use Ultimate over Business in a work environment unless you want to watch movies, record TV, etc. Of course you can install all of those multimedia features via a third party application, one that actually works correctly.
Vista Enterprise is not an option for someone unless they are buying large volume licenses (I do not know the magic number).
Remember you can purchase a Vista Business lic and downgrade it to Win Pro as long is it is installed with the OEM version. Unfortunately that is becoming the only option to still use Vista Business but there are Windows XP machines out there. I saw a whole bunch of Win XP home machine, new at walmart today (of course they are single proc systems).
I really hate this Franken-Vista mess of different OS versions with different features. I can not recall an example of where a business actually removed from the market a product that customers wanted (win XP) in order to force a product they did not want (Vista). Vista has done more to sell Macs than any marketing that Apple could come up with.
