If you cannot remember the passcode, you will need to restore your device using the computer with which you last synced it. This allows you to reset your passcode and resync the data from the device (or restore from a backup).
If you restore on a different computer that was never synced with the device, you will be able to unlock the device for use and remove the passcode, but your data will not be present.
You may have to force iPad into Recovery Mode
Options for when an iOS device gets locked because of forgotten password:
Restore (and reset password) on your device by connecting it to the last computer to which it was connected:
iTunes: Backing up, updating, and restoring iOS software - http://support.apple.com/kb/HT1414
If you cannot connect it to the computer to which the device was last connected you will have to use recovery mode to completely reset the device, losing all data:
iOS: Unable to update or restore - http://support.apple.com/kb/HT1808 - recovery mode (e.g., cannot connect to computer last used to sync device, iTunes still asks for a password)