Yes the warrenty is void once you jailbreak. Also the only way to perform a full restore is to connect it to a computer with the cable.

However you can perform the following steps and they probably wont be able to notice at the apple store if it was broken and will replace it.

#1 go to settings
#2 go to general settings
#3 go to reset at the very bottom
#4 select Reset all content and settings

This will reboot the iPad and you will see the white progress bar as it clears everything off of the iPad. Then it will come to the screen saying "connect to iTunes" just like how it came from the factory. They will be unable to use the iPad unless they get the cable connection to work and connect to iTunes. Even then it may not be jailbroken any more.

One learns something every day. Thanks mike

I am assuming you want to restore the iPad. If you had the same problem, then you would just be handing the iPad in to Apple, which is what the OP wanted to do and that's why he was given this advice. If you are handing it in to Apple for replacement or whatever, then just hand it is now...

If on the other hand you want to restore the iPad to working operation yourself, you need to enter DFU mode. I wrote this guide to help you...

http://www.ipadforums.net/ipad-hacking/27489-dfu-mode-dummies.html

When you connect to iTunes you will then be prompted to restore. Using DFU mode is the ONLY way you will get it to work so you must use this method.

So if all you want to do is return it to apple, then just follow frenchroll's instructions above. This will render your iPad inoperable but there will be no way for Apple to tell that it was jailbroken. But be clear. You will not be able to restore it yourself if you change your mind, unless you can get it into DFU mode and recognised by your PC again...
It is the best and only thing you can do if you intend to return the device and cannot restore it yourself. This is the only scenario I would recommend doing this for.

Well, I have never heard of "Erase All Contents & Settings" not rendering a jailbroken iPad inoperable, which is what you are trying to do. For the avoidance of doubt, that is the ONLY reason I am recommending you do this. Normally you NEVER take this option with a jailbroken device, but you have stated you want to return the device to Apple and it is jailbroken, but you can't restore it on the computer.

Usually, once you press the "erase all content and settings" button, the device should be stuck at the Apple logo and even if you reset the iPad it should still hang there in an endless loop. I am amazed you are able to reset and get the iPad to reboot into a jailbroken state. How long did you leave it stuck at the Apple logo before you reset? You should leave it for a good few minutes, and not reset immediately. Perhaps it is because you tried to remove components of the jailbreak with Mobiile Terminal, I don't know and it is impossible to troubleshoot.

The only option open to you now is therefore to fix the problem with your PC. You need to put your device back into DFU Mode and then get the drivers to install correctly. You must have interrupted the process before (it is easily done).
The easiest way to do this is to plug the iPad into a DIFFERENT USB port than the one you are using ONCE the iPad is in DFU mode (not before). You know it is in DFU mode if you followed the instructions and the screen is black.
If you are using a USB hub, then don't plug it in there. Plug it directly into another main USB socket on the computer and allow the DFU driver installation to complete. iTunes should then recognise the device and allow you to restore it to factory settings.
If you cannot solve the problem with your PC, then you will need to try any friends that has iTunes installed (it will not mess up their PC, as long as you connect the device when it is in DFU mode and allow the drivers to install). Again, iTunes will prompt you to restore the iPad and it will set it back to factory defaults on 4.3.3.

Sorry I can't be of any more help.

Good for you

I would suggest following the guide here - http://www.ipadforums.net/jailbreak...lsafe-method-restore-current-version-ios.html

You can probably skip Step2, since if you have never had an iPad before it won't be relevant, but make sure all the others are followed. You should end up with 5.1.1 freshly installed on the iPad.

Good Luck!