The PAYG iPhone will still be SIM locked to the O2 network.
True.
Long story short is that no matter anyones outrage, sense of entitlement or belief that Apple owe them a carrier agnostic iPhone in the UK, it won't happen.
The best one can hope for on the UK PAYG front is an O2 PAYG iPhone or, if the rumors prove true, an Orange PAYG phone by the end of the year. Rest assured that whatever the carrier, whatever the terms of sale (18 month contract or PAYG) that Apple will get their money, the carrier/retailer will get theirs and that the device will be simlocked.
The only reason any other country might be getting non-simlocked iPhones is down to said country having laws preventing the sale of handsets being locked to one particular network as part of a contract.
I seem to remember that the brief appearance the O2 PAYG iPhone page made a few weeks back aluded to a minimum top-up spend per month for a fixed number of months in addition to the cost of the handset. Feel free to correct me if I don't recall this correctly. I seem to remeber there not being a huge amount of different in monthly contract (£99+(18x£30) and the PAYG option (£handset+6 months minimum top up spend).