Skype works just as well on the iPad as on the iPhone, and yes, the iPad has a build In mic.
The only thing though Is that skype not Is optimized for such a large screen as the iPad, but just press the X2 in the lower corner.
Or try use "FullForce" that can be downloaded from Cydia or Rock, FullForce forces the apps of your choice to fill up the entire screen, this do not work on all apps.
Yes, I used it when I missed a train connection to call a relative just by speaking into the iPad itself. It works over 3G as well as WiFi.