Hopefully I'll post a section or two a week for the next few weeks to whet your appetite for Mel8ourne!
So on to our first subject:
What to attend?
In 2008 the full conference schedule is from Monday to Friday, with an Open Day on Saturday.
The "main conference" runs from Wednesday to Friday, however don't think that the miniconf's (Monday and Tuesday) aren't as important as the main conference, if there's one that covers your day-to-day work (eg, Debian or Fedora for sysadmins, perhaps Kernel or MySQL if you dream in algorithms) you could find it more valuable then the main conference.
The Open Day on Saturday is an opportunity to talk to both major and minor open source projects, some of the companies behind them and the Open Source interested public.
And see people like this do strange things while sober.
Many people find the best time to arrive is late Sunday afternoon so they can register (and pick up schwag :-), settle in and then socialise for the evening without worrying about missing the start of their favorite miniconf.
As for when to leave Sunday is best as it lets you attend open day and gives you another evening for Socialising, but if you want to do touristy things or visit long lost relatives you should do it before or after the conference as conference activities run from around 8am to after midnight all though the week, and the most interesting ones are the ones not on the program.
If you can't afford to be away from your job for the entire week don't just go to the main conference, check the schedules of the miniconfs (which should be posted soon) and the main conference program to get the most bang for your buck. Usually speakers at the main conference will attend a relevant miniconf and most are happy to answer questions if you can't attend their talk.
This year the second "Foundations of Open Media Software" will be held in the week before the conference, if you're a developer working with multimedia you should seriously consider going.