Welcome to the forums k7sle,
So Ubuntu lands works a little different than the Microsoft world...
Ubuntu releases new versions every six months, and supports those releases for a relatively short amount of time with patches to critical bugs, etc. There are also LTS releases, which have three years support for the desktop version and five years for the server version; of course daily security updates as needed. The wiz-bang new features go into these 'in-between releases' and if their good enough, make it to the LTS. (Well that's a given for the most part, unless they would be difficult to support for 5 years etc.)
7.10 was officially released on october 18th of this year, essentially just one month ago...
7.04 is only supported till 2008, and 7.10 only until the start of 2009 (no difference for desktop vs server versions on those dates).
6.10 is also just till 2008. Most just don't have time to upgrade platforms on a mailserver every year, and companies just don't have the time to test every new instance; so 6.06LTS to 2011 is nice & ensures the longest help for everyone. For starters, on a mailserver you don't need the latest desktop UI...you get the idea, you're not new to linux, sorry for being repetitive!
As you can see by the wiki doc it really doesn't take a whole lot besides tricking the installer (unless you go 64bit).
We do try to keep the wiki updated with as many tips as possible, so if you find something new let us know in the forums or even add it to the article/talk pages yourself!
If you're curious, planned supported platform list for 5.0 is here:
Zimbra Product Portal - Release 5