So I ran that second script which counted the number of times each header popped up (using something that looked like my Widefinder script, amusingly) and put the text file of the results up here if you're interested.
It's not telling me much, but I figured I'd share. It looks like a couple thousand records were missing dumps, probably because they were too big for the size of the text field I was dumping in - I'll have to up that for the edge cases. There's a bunch of PHP/Apache headers in the beginning that don't have anything to do with mobiles. Also, there's enough examples that the numbers give a good general idea of the common headers, but these are coming off page views so if a really enthusiastic bunch of Nokia users hit the site a lot, they could have skewed the numbers a bit - probably not, but you get the idea.
* HTTP_ACCEPT wasn't included in all of the requests (200k were missing!) So much for relying on that header to exclusively designate a mobile (i.e. does it accept vnd.wap.*).
* 5.2MM requests had HTTP_COOKIE. Does that mean that 1.6MM requests were initial entries?
* The numbers fall off quick. I ordered the list by number, and the 40th entry out of the 599 is HTTP_X_NOKIA_CONNECTION_MODE which only has 1.4MM entries
I was talking to Tarek on #mobitopia, and he was confirming that trying to extract any sort of individual unique ID is going to be impossible, so I'm going to have to come up with some sort of combo of headers, which if unique, indicates a unique user... If you have any suggestions along these lines, I'd be glad to hear them.