[webkit-dev] identifying a unique iPhone?

[Scott]

Hopefully this is the proper list to post to:
  I am writing an iPhone web application, and know how to tell that I am 
talking to an iPhone, but is there a way to uniquely identify a specific 
iPhone?   For example, some header or something I can get with either 
server side or browser side code and store on the server side so in the 
future I know which iPhone this is?

I know I can implement a cookie style log-in system, but I was hoping 
for some value I can just get and use.

Thanks,
Scott