Firstly, I'm not sure if this counts as an exploit, although I believe it does which is why I set it to private. I apologise if I did it wrong :/

With the new world borders you can still interact with the world outside, which normally wouldn't be an issue. After a bit of testing though, I discovered this can be used to get you outside of the border.

I realise that the world borders are in heavy development and this probably won't be possible in the future, but I thought I'd report it just in case.

I tested around with mine carts and boats and, as expected, they cannot move through the boundary. Despite this though, carts can still be placed on tracks out of the boundary. This does not work with boats. Upon entering the cart and exiting it again, you are stuck in the boundary block next to the cart.

Also, when leaving boats and carts at the edge of the boundary you exit to outside it, although due to it only being 1 block out you can usually walk back out.

I've included a little video of the cart behaviour. Apologies for the terrible quality of it. I've still to find some proper screen recording software. File size was too bit go put as an attachment, so I linked it via dropbox instead.
http://tiny.cc/87uvex

Sorry if it's already been reported. As far as I can see it hasn't. Hope it helps!