Same thing has been happening to me for about 2 weeks now.  Bought my N7 from Best Buy 8/15.  It'll go to sleep and sometimes it'll wake up if I plug it into the charger.  Sometimes not.  If I hold the power button down after about 10-15 seconds the Google logo and four color flashes come on...but then it goes back to sleep.  I know it's working because when I receive an email a "bing" noise occurs but I have to read it on my Blackberry because my Nexus won't wake up.  I started with BB.  They escalated me to Google.  Google passed me onto ASUS.  It took me three days to get through to ASUS.  Then they told me to fax them a copy of my purchase receipt and that they'd get back to me within a week telling me where to send my unit in to get REPAIRED...provided they accept my case!  ASUS kept getting all bunched up about what third party software I might have installed.  Guys...it came from the Google Play Store.  And we did a system reset on it and even before any new software was put on the same thing happened.  Is anyone listening?  Anyone at all!  Because it's not apparent to me right now.