The Google Nexus 10 comes out shortly, and is the first device to use xxhdpi resources. It sports a display density of about 300 DPI (according to the Nexus 10 website and t
A set of four generalized sizes: small, normal, large, and xlarge Note: Beginning with Android 3.2 (API level 13), these size groups are deprecated in favor of a new technique for managing screen sizes based on the available screen width. If you're developing for Android 3.2 and greater, see Declaring Tablet Layouts for Android 3.2 for more information.
A set of six generalized densities:
ldpi (low) ~120dpi
mdpi (medium) ~160dpi
hdpi (high) ~240dpi
xhdpi (extra-high) ~320dpi
xxhdpi (extra-extra-high) ~480dpi
xxxhdpi (extra-extra-extra-high) ~640dpi
From developer.android.com : http://developer.android.com/guide/practices/screens_support.html
xxhdpi was not specified before but now new devices S4, HTC one are surely comes inside xxhdpi .These device dpi are around 440. I do not know exact limit for xxhdpi See how to develop android application for xxhdpi device Samsung S4 I know this is late answer but as thing had change since the question asked
Note Google Nexus 10 need to add a 144*144px icon in the drawable-xxhdpi or drawable-480dpi folder.