please read my solution some time ago, a near similar question:
Working with USB Devices
Especially following great CP-Article:
A USB Library to Detect USB Devices
In addition there must be some hardware-id which is stored/recognized/integrated drivers of win (sometimes through windows update), if iphone was plugged in.
Unfortunately I'm not 100% sure, but I think PNP-ID/Device ID should be always the same id, you're right, serial number is unique.