Another stupid question....
I clicked Buy Myki PASS, and then where you enter your details it says which zones....err, why are there TWENTY zones, last time I checked there was zone 1= Melbourne, Zone 2= outer suburbs....
I like this new one, I HATED carrying loose change around and losing like 5 cents making me unable to buy a ticket, grrr. This one I just recharge 
I've heard the bus and tram myki system will not be up and running for awhile because it is currently encountering technical issues. Even the train system seems to be being "rushed" out before the end of 2009.
For those unsure how the system works, assuming it works like Singapore's (very efficient) MRT system, your myki card is similar to a debit card that you use on public transport. The card can be topped up from specified machines located at and around stations. (usually you place the card on the machine scanner then insert how much you want to top up into the machine). You do not throw the cards away after using them once. They hold a certain amount of credit and when that credit runs low/out you are expected to top the card up again.
When you enter a train station or bus/tram you will scan the card on the scanner in a similar way to the current metcard system. The difference is you will also need to scan your card when exiting the station you are travelling to, or alighting from the bus/tram. This second scan calculates how much to deduct from your credit based on your travel time/distance. If you do not scan your card the second time then you will be charged the maximum fare (Not sure what this is).
While all this scanning may sound like a hassle, the actually scanning of the cards is very quick and easy. If the scanners are anything like in Singapore, then they can usually detect the card through a wallet or even a bag. This means you don't need to take the card out of the wallet to scan it, you can just place the wallet over the scanner and it should detect your myki card.
Note I may be wrong on some/all of the above as I'm just assuming the system will work the same as other nation's that have similar smart cards.
Also, the metcard system will be running concurrently with the myski system for 6 months I believe. I would probably stick with the metcard system until the work out the bugs with the current myki system. There have been numerous reports that the machines are failing to calculate fares and are often charging the maximum amount when they shouldn't be.