Amazing what you find when you start searching even more. I'm really surprised Google Alerts never picked some of these up before and alerted me on them!
Detecting Rouge Wireless Devices via DHCP Fingerprinting:
Microsoft - 2011
Appears to be a similar one, but not sure of differences right now.
Microsoft - 2007
System and Method for Resolving OS or Service Identity Conflicts (using SMB, DHCP, etc)
SourceFire - 2011
So it looks like a few other places have put some patents on DHCP fingerprinting in the past few years also.