Whew, caught up again! Just to throw my 2p in on coverage
In order of 3G coverage from best to worst:
Three > Orange > Vodafone > T-Mobile > O2
I'd agree with that order for 3G and/or urban coverage.
There actually seems to be a lot of site sharing going on with 3G, probably because it has a shorter range and so needs to be hidden away somewhere within the town in a way that doesn't upset NIMBYs (very difficult) rather than being out of sight and mind in a field or out-of-town industrial estate like 2G masts. Getting planning permission for antennas in urban areas is tough, so the networks seem to agree that once they've found somewhere they all go for it. In our case, new flag poles keep mysteriously appearing on an old cinema building.
So coverage/range theoretically would be about the same, but the difference is in how quickly the networks put their equipment in. In our case T-Mobile have beaten Vodafone to it and they've always seemed to have a close relationship with 3 (long before the acquisition) so maybe I'd swap those 2 around but I'd definitely agree with 3 being first and O2 being last.
The order also seems about right for the motorways, with 3 being pretty much non-stop coverage and O2 seeing 3G maybe once or twice driving down most of the M6. The others are somewhere in the middle.
OTOH for 2G/countryside coverage:
Vodafone > Three/Orange > O2 > T-Mobile
Based on various manual network scans around the lake district and fells. Not sure about the rest of the country. I suspect O2 2G probably has quite a strong presence further south, they've always seemed to lag behind up North though.
Vodafone seems about the most balanced of the networks, but on the iPhone my priority is urban/3G coverage so I go with 3. I carry a small Vodafone (Asda) mobile for walks.
I'd probably consider Vodafone if they got 3G in this town and if 3 got a bit crap/annoying.
Does anyone know - for sure - that the iPhone 4 that is sold on PAYG is definitively locked to a carrier?
Unfortunately no. A handful of people think they do, but it's not been definitely stated in writing anywhere so currently it's anyone's guess. We'll know by Thursday anyway.
I would think through the networks it'll be locked. It's just the way they're used to doing things. When it comes to contracts I'm not really sure why - they're still getting the money for the contract, and heck if the person uses the phone on another network it takes the strain off theirs while they still get a profit! But they seem to enjoy the control over customers.
This I think would include buying it through Apple and specifying a network. They'll sign you up with an iPhone-specific contract or special iPhone PaYG deal where you get 12 months of 'free' internet, and in the process of activating the phone, it'd lock it to the network. Remember the way locking works on the iPhone is through Apple's servers, rather than the traditional hardware way of most phones.
SIM-free it is stated in black and white that it'll be unlocked (that's the definition of SIM-free anyway, but it's also mentioned on Apple's site). This could be the only occasion where you get it unlocked, and of course you pay the full price for it rather than £480+free internet etc.