This is a common fault, caused by buggy iPhone software...
Check out this thread on Apple:
http://discussions.apple.com/thread.jspa?threadID=1600894&tstart=0
The thread has been locked for some reason...
I've experienced 6 'bricking's' so far, (iPhone stuck on Apple logo, only able to force it to restore) these always seem to be caused by interuptions(eg. phone calls etc) during an install/update of an application to the iPhone. It doesn't always lead to an immediate 'bricking', usually the device appears to resume installing after a delay. The first sign of a problem is backing up the phone goes from taking 2 minutes to 4 hours+ every time the phone is connected, I believe this is because the phone is failing to back-up a corrupt application (the App interupted during intall most probably). These back-ups are always corrupt when I try to restore from them so they always fail. Eventually after anywhere up to a day, the phone will 'brick' after the install interuption event.
I think it probably effects every iPhone, but is only noticeable if you have more than a dozen or so Apps, because the less Apps, the less times you have to update/install Apps, hence the lower risk of 'bricking' the device.
Unfortunately despite greatly reducing my number of Apps to a couple of dozen, my girlfriend seems to know exactly when my iPhone is updating Apps and has managed to call me twice during the five minute events!
The lesson is, currently to make brickings less likely, install less apps

to avoid brickings completely, install no app's or avoid updating.
Very bad bug that Apple is doing everything it can to avoid publicly revealing, as the App store is so important to the devices success...