After installing the IE 6 preview, I had the following registry key added to my Win2k Pro box:
\\HKEY_LOCAL_MACHINE\\SOFTWARE\\Microsoft\\MSLicensingWithin this key are 2 subkeys: "HardwareID" and "Store"
Under the "HardwareID" key is a "ClientHWID" binary value entry, containing what I'll assume is a hash key generated off my system's hardware.
Under the "Store" key is another key named "LICENSE000", which contains four binary values named the following: "ClientLicense", "CompanyName", "LicenseScope", and "ProductID".
Make of this what you will, but I wouldn't be surprised to see MS start sneaking code like this in through hotfixes and service packs. It may become rather difficult to avoid their registration scheme.