I Had This Problem After I upgraded to 4.3.5...
apps would not update, neither could I buy new apps or books or music from iTunes. Here was my problem: Everytime I tried to update an app, or buy a book in iBooks, or a song, I would get the Terms and Conditions Page where I would have to agree to the terms and conditions, then it would take me to a note that says I must have Accept Cookies enabled in Safari with Visited Sites chosen.
Even if this was the way it was in Safari and after I powered down the iPad, and after reading endless notes at the Apple support website, after you have powered down the iPad, and the problem persists, the ultimate recommendation was to do a Restore.
It took an HOUR to restore my iPad. But it was worth it. My iPad updates apps again, I can buy books, tunes, and videos from Apple. Side note: this was not a problem with Amazon Kindle or Nook--just Apple and iTunes.