http://www.ifixit.com/ - best site for iPhone parts apparently if you have an issue like this. Full walk through of what to do in many instances as well
iPhones and Apple products in general are mainly sealed units. Macbooks you can replace hard drives, ram and that's about it. Your optical drive goes and you're kind of screwed for example.
The iPhone is no exception. It has it's flaws and I'll never deny that BUT it sells for a tonne of reasons - one of the main ones being that everyone can use it. It's just about the most intuitive device you can buy and the shear simplicity and accessibility of it sells it. It doesn't baffle a user with tech terminology and ask you to read the manual to set it up. Sure this may annoy those wanting to delve deeper into the software/hardware and muck about with it but many don't want to do that.
I admire the ease of using it - the interface is massively better than most other touch screen phones I've used, the responsiveness of it for example is another sticking point for me over other phones. The OS doesn't delay in actually operating the way many Sony Ericsson smart phones do and there's no awkward stylus needed, which I've also seen on many other phones.
The flaws - camera is crap, which is fair enough. 2mp (in the case of my 3G) on a modern camera phone is indeed rubbish. But compared to other camera's of the same rating it actually stands up rather well in my opinion. I do wish it had more options to edit pictures and the like but then that's what apps are for
Other things like bluetooth capabilities are indeed idiotic of Apple. I think it's to do with their whole file sharing paranoia, linked to iTunes and all that. It's something users have been crying out for them to change, thankfully it's something a firmware update could change. I also believe that if you jailbreak the iPhone you can use bluetooth a lot more freely.
For the record as well, I didn't actually want to get an iPhone in the first place. I was going to get a Samsung i8510, but couldn't on o2 for the money and the n96 I'd heard was slow and also expensive so got an iPhone and the contract that came with it. Oh yeah and it was FREE from o2
I'm also on my fourth handset BUT I took it to the Apple Store in Glasgow each time and they replaced it there and then rather than faffing about - when did you ever get service like that direct from one of the major networks?
The only other handset I'd consider away from an iPhone is a decent Android powered smart phone - something from HTC for example though apparently Windows Mobile on the HD2 is pretty decent.