The info I have found: 320dp: a typical phone screen (240x320 ldpi, 320x480 mdpi, 480x800 hdpi, etc). 480dp: a tweener tablet like the Streak (480x800 mdpi). 600dp: a 7” tablet (600x1024 mdpi). 720dp: a 10” tablet (720x1280 mdpi, 800x1280 mdpi, etc).
But its coming from 2011. Now is 2014 and there are 2048x1536, 2560x1600, 1920x1200 tablets and phones. Are they still 720dp width or it does exist also 800dp, 1024dp or any other? That remains unclear to me because all the discussions I have found are from 2011 year, when there were no such devices on market. If it depends on manufacturer, can you please share info about Sony Xperia tablet Z and Samsung TabPro 10.1 width in dp?