I looked at Skype garage blog and there are some mentions there: (we are talking about Windows Phone 8 version of the app)
For Preview 2.0 (12th November 2012):
you'll get all of the Skype basics in the new, modern design along with many of the new features such as integration with the People Hub and a combined contact list with buddies from Windows Live Messenger. Additionally, you'll start receiving some call and message notifications - even when the app is closed and in the background.
For Preview 2.1 (2nd January 2013):
In this release we have worked on improving reliability and performance of the application, especially around background notifications and calling.
We have also temporarily disabled the People Hub integration as it was causing issues with device stability. We are working with the Windows Phone team to resolve the issue and to enable the integration again in a future release.
The main thing here is that it is only preview. But they are evidently working on it, so just follow their blog or wait for future updates.
And to answer your question completely - what is meant by integration?:
The main thing in integration is the behavior when the Skype is in background.
Windows Phone users won't have their Skype client going offline once it is suspended. In fact any VoIP client on Windows Phone 8 will be allowed to work in the background.
And it is much more metro-styled now and therefore much nicer :) .