Well, If you could get to the underside of the skin, you could do something like a bicycle tube repair (You could do this from the outside, but you would get an ugly patch on the outside of Pleo). I would use contact cement, and a piece of rubber as a patch. You can get contact cement at your local hardware store, and you can find appropriate thickness rubber patches in old bike tubes, or something similar. (I'm sure you could probably skin graft from another Pleo if you really wanted, but thats a little drastic).
Let us know how it works out.
This is an undocumented field! I haven't heard of any Pleo repairs yet, although I expect it will be more important going forward.
Pleo is quite complex internally. The neck is a delicate mechanism, and I'm not surprised it broke eventually. The only way inside is to completely remove Pleo's skin. Once you remove the skin, you can't really put it back on. So if you were to disassemble and fix the problem, you would have a naked Pleo. Which might be quite cool!
If you're motivated, cut it open and post some photos of the broken neck. We'll see if we can suggest any tips.
I mistakenly thought he had a broken neck. It turns out the issue is with the a split in the skin, so Brett's answer is better.