The Panasonic sites states that the camera comes with a built-in microphone and that the unidirectional microphone (the one shown on all the pictures) is optional.

How much will that microphone cost?
Is it worth it?
How good it can be?
What would you buy instead?

Personally, I think if you're going to spend any money at all for an exterior mic you'll usually want to go ahead just get a really good one (better than the upgrade Panny's offering) that you can have for 10, 15 years over the course of three or four cameras. I use a Sennheisser shotgun on the camera and I can also use it on a boom--I had to pay a few hundred dollars for it but the sound is perfect and with a little luck, kids who are little today will be drinking age before I have to replace it--and you can usually get a slightly discounted used one of terrific quality on ebay, this site, maybe even Craig's List, etc.

Well, for HD cameras you need Panasonic's HD microphone.

Standard mics won't work nearly as well because of the new format's higher defnition...

... if you believe that, then I should be a salesman! :-)

Seriously, I was using an AT 897 shotgun but just started using an AT 4051 unidirectional mic on my Sony PD-170.

The AT 897 was selected because it didn't require phantom power and I was using it on a Canon XL1s.

But when I started shooting with a Sony PD-170, which does have phantom power, that opened up a wider selection of mics, and I decided to use the 4051. It's better at rejecting noise from the rear of the mic and has options for unidirectional and supercardioid pickup patterns. Fidelity is wonderful, too.