Todays update on the O2 site was certainly worth the wait.
Apparently, they're short of iPhones! Well I certainly never saw that coming.
Some stores will be opening early, some not. The ones that aren't opening early will be open at their normal hours - whatever they are.
You might be able to upgrade in an O2 store or a CPW store, or maybe just in O2 stores, or possibly not at all - not entirely sure as there seem to be pages which suggest any or all of the above, and they all link to one another. You'll need ID in an O2 store, but there isn't any info for CPW (you could assume you'll also need ID, but then you also have to assume they'll sell you an upgrade in the first place).
No online ordering - that would be too easy.
Apparently all this is completely unprecedented too - nothing else has ever been supply constrained.
I'm sticking to my plan of not bothering with O2 until they get their act together. If it turns out that I can walk in to a shop and buy one with minimal hassle, I will. If I have to queue, and their online store doesn't start taking orders, I probably won't bother getting one at all. If O2 doesn't want my money I'm more than happy to keep it and/or give it to some other company instead.