The Complete Guide to Using the iTunes Store
iTunes Store: Associating a device or computer to your Apple ID
Can’t connect to the iTunes Store
iTunes: Advanced iTunes Store troubleshooting
Best Fixes for ‘Cannot Connect to iTunes Store’ Errors
iOS: Changing the signed-in iTunes Store Apple ID account
Try this - Reset the iPad by holding down on the Sleep and Home buttons at the same time for about 10-15 seconds until the Apple Logo appears - ignore the red slider - let go of the buttons. (This is equivalent to rebooting your computer.) No data/files will be erased. http://support.apple.com/kb/ht1430
This works for some users. Not sure why.
Go to Settings>General>Date and Time> Set Automatically>Off. Set the date ahead by about a year.Then see if you can connect to the store.