I would say by asking for a refund (ignoring the fact that you were asking for more money back than you'd actually paid) it would be assumed that you that cancelled the contract...
I reckon this is the best argument they've got. But they only cancelled the second time I asked this.
(Roughly paraphrased from memory)
August 9th:
Me: I was told to call if I didn't hear anything by now. I've not heard anything.
CS: I'm sorry about that, is there anything we can do instead?
Me: I'd take a full-price (let me be clear, that's the pre-sale price) refund as an alternative.
CS: Um, let me call you back.
(I get a call back later - the only time I'm ever called back - promising it will arrive in another week)
August 17th:
Me: I was told to expect a call by now about my order, but I haven't.
CS: I'm sorry about that, you'll receive a call back in 24-48 hours.
Me: Look, I'm starting to wonder if you're ever going to get these things in, and if you're not I'd take a full-price (let me be clear, that's the pre-sale price) refund as an alternative. Perhaps there's a supervisor I could talk to?
CS: Sorry, that's not possible.
Me: Fine, I'll call back later.
(Cancellation email arrives shortly afterwards.)
I'll add:
August 17th, later:
Me: You cancelled my order! Why?!?
CS: Because we don't have any stock.
Me: But that's been true for the past eleven weeks, and you've been saying there's more stock coming! Why cancel now?!?
CS: Because you asked us to.
Me: No I didn't!
CS: Well we can't reinstate it.
And of course I should mention:
August 19th: (possibly 18th or even late on the 17th, I didn't look until the 19th)
Website: Look, we have the D700 in stock!
Now I want to ask, since I don't remember the last time I actually cancelled an order by phone - sellers aren't normally so quick to assume, are they? They ask you so they're sure, "to confirm, you wish to cancel you order, correct?". They might ask if there's something else you want to buy from them instead. I'm pretty sure that any customer service rep who cancelled orders at the earliest opportunity would be out of a job pretty quickly.