Couple of updates from me...
Firstly, rang my local carphone warehouse and they were taking a list of pre-orders. I was the first person to put my name down for a 32GB, they had 5 already for 16GB.
More interestingly, I rang O2 to get a PAC in case I wanted to switch networks, and the guy on the phone said that before I went out and bought an iPhone on Thursday he would "strongly advise" that I first rang the retentions line and tried to get a deal off O2. Apparently they won't know until Thursday itself, but are fairly likely to be offering extra minutes / cheaper contracts to people who threaten to switch. He also said that they would send out the new phone by next day delivery if I did take a deal. He kept saying that this wasn't confirmed but it was a "very strong recommendation" that I should ring back on Thurs.