Can you reattach an SRAM powerlock? An advert for their PC-1031 chain on chainreaction said the Powerlink Powerlock was one time use only; you couldn't attach the chain, detach it and then attach it again without a new Powerlock.

I understand Powerlocks probably only guaranteed the first time, but in practical terms does it really work that way? Do chains usually snap if you try to reuse the Powerlock?
For 9 speed or below, SRAM sells the Powerlink, which is a reusable way of opening and closing a chain. You can open and close these a few times, at least for the life of a chain.

For 10 speed and above, SRAM sells the Powerlock, which is a one-time use way of closing a chain. It is not designed to be opened and closed, and it is not suggested that you re-use it. 

Do people re-use Powerlocks? Sometimes. Do they get away with it? Fairly often. Do they come apart? Also reasonably often. Would I recommend it? Of course not -- they're cheap, just buy a new one each time. It's generally going to be worth the potential costs of dealing with chain failure when you're out riding. 

If you want to use a quick link for a 10 speed+ chain thats able to be opened and closed repeatedly, try the KMC Missinglink for 10 speed chains or the Wipperman Connex link. Both are designed to be reusable. (*)

(*) Officially, chain manufacturers don't recommend mixing brand X's chain closing mechanism with brand Y's chain when X!=Y, but people have generally found it safe to mix quick links between manufacturers. So, with this in mind, I'd still recommend the KMC or Wipperman if you want to detach and reattach the chain repeatedly.