Sorry I can't yet answer most of your questions (I'll answer what I can when I get back), but I do have to say that unless they figure out how to unlock them, the initial iPhones (the ones that come out in June) will be locked to CIngular, which you won't be able to use in the UK, unless you don't mind paying ridiculous roaming rates :-\
As far as third party apps, right now i seems that's not possible. Even if you could get VOIP though, it would really only work over wifi, or through a bluetooth dialup connection. EDGE isn't fast enough to support VOIP.
Will the iPhone accept third party and /or Apple apps that add functionality, espec things like PDF/Exchange/push Email/VOIP functionality?
It does have Yahoo Push IMAP built in, but it's Yahoo...
Don't know about PDF or Office support.
Not sure. I hear many people saying it does, but I can't actually find any evidence it does.
Does the Bluetooth 2.0 implimentation drive stereo wireless headphones (A2DP)?
You better hope so. I doubt it'll be the most durable thing out there, especially since it's so thin, but the chrome backing (I'm assuming that's what it is, that's what they use on the iPods) should help. I'm sure there'll be lots of cases available to protect it.
Will it be rock solid (like my Powerbook)?
Personally, I don't think a virtual keyboard will ever match up to a hard keyboard, but the speed will probably depend more on the user's typing skills and adaptation to the iPhone's keyboard.
Will the keyboard be fast enough to enable new texting records to be set on it (like the Treo 650 did) (BTW my Treo keyboard is FAR from ideal)
No one can say yet, except Steve Jobs.
What about signal strength and call volumes? (major Treo weknesses)
I'm sure Apple has made room for this. Don't kno about extra memory or 3G (I highly doubt most people would go for the idea of an external cellular radio or a big memory dongle hanging from the iPhone), but I'm sure other things can be done, such as an FM tuner, video out, etc.
Can any functionality (extra memory, 3G etc) be added via the main connector?
I'm sure EDGE is already in the UK. It's actually quite old compared to what you should be using. From what I've heard, UMTS and HSDPA are available in the UK, both of which are much faster than EDGE.
Does EDGE exist yet in the UK (no one seems to know)? Will it kickstart EDGE in the UK?
I think this will eventually become a problem. Like all rechargable batteries (at least from what I know), the iPhone's battery will eventually start losing its charge and will need replacement. I'm sure you should be able to get at least 1.5 to 2 years of usage (well depending on how well you maintain your battery) before something like this should happen, but in any case, if a battery does need replacing, you'll have to send it to Apple in order to get it replaced. Third parties may do it or you may even find out how to do it yourself, but that will probably void the warranty on it.
Having just replaced the battery on my son's iPod mini for £10, should I be worried by the 'built in' battery?
Looking at the specs, it doesn't seem to have IR built in. The only way you might be able to add one (that I can think of) is through the Apple connector on the device, but I highly doubt you'll see any addon IR transmitters.
Does it have / can I add a FULLY POWERED IR transmitter?