Just something that occured to me when I plugged my iPhone into my PC to charge up....

I have a couple of app's on my iPhone (VLC Player, a comic viewer and a couple of wifi locators) which are no longer available from the app' store.

My phone was reset a while ago, after it was unlocked from the Orange network, and I thought that'd mean I'd lose all the unavailable app's.
However, the next time I plugged the phone into my PC iTunes just restored all my app's back onto the phone, including the ones that aren't available any more.
Yay!

My question is this: Does the phone reinstall the app's from data stored on my PC or does it suck the data off the internet somehow?

It just occured to me that if it's reinstalling the app's from data stored on my HDD I guess I'd better start backing-up that data in order to avoid losing those app's.

If that IS the case, anybody know what files I need to back-up?

Alternatively, does iTunes upload a list of all my app's to the interweb somewhere and then re-download them when neccesary, INCLUDING ones that aren't on the app' store any more?

If you're syncing with a cable and your PC then it's restoring from the PC.

If you're syncing with iCloud, then your apps will be stored there and you can restore from there.

It's via a USB cable.

Any idea what files I need to back-up so I could restore my phone after, say, formatting my HDD?

I suppose simply formatting my HDD while my phone still had the app's on it then synching after reinstalling iTunes would create a new restore point on the newly formatted HDD which included all the app's but I'd rather KNOW I have the data backed-up somewhere.

No, don't presume the phone will restore back to the computer. It can be done but things can get confused.

Basically you need a recommendation of a PC backup solution, so maybe you should post in the PC forum.