So should my layout have this in the line
ads:adUnitId="ca-app-pub-1234567890123456/1234567890"
Yes, if you are defining your Adview via XML.
Is the line in the activity still required, and will it be the new format?
adView = new AdView( this, AdSize.BANNER, "ca-app-pub-1234567890123456/1234567890"
Construction of the AdView in your Activity is only required if you haven't defined it via XML. Ie you either construct the AdView in XML or in code.
And yes, the AdUnitId in code is the same as that used in XML.