It's only integrated on the europe devices. With webOs 2.x apps can integrate seamlessly into messaging (this is called "synergy plugins"), that's what the palm skype app (and also the facebook app) does. So it's theoretically possible to install the skype app on "any" webOs 2.x device.
From my perspective the problem could be packaging the app... but maybe there are special things build into the system for skype.
I have a UK Pre 3 on T-mobile here in the US and Skype just works as a calling option. This includes video calling; (though I can send video, I have not yet successfully received video: it's just a black image). A Skype account has to be added in Accounts and there have to be Skype addresses in Contacts.
I installed it on my ATT pre3 and it winds up integrating the same as my UK pre3. I'm guessing it was not on there by default because ATT wouldn't allow it, but, since these aren't ATT's phones, HP put it in the catalog.
Original Palm Pilot+modem > Kyocera QCP 6035 > Kyocera 7135 > Treo 650 > HTC TyTN (ugh) > Palm Centro > Nokia E75 > Nokia E72 + iPad2 > HP Veer + TouchPad + UK Pre3 + AT&T Pre3 > iPad2 + ATT Pre3 + Nokia N9 > Galaxy S3 > Pre3 > Nexus 4 > Blackberry Q10 + Galaxy Note 8.0