If you've seen the field guide in the Halo 3: ODST project page, you should be able to tell that the end of the service number contains the soldier's initials.
[quote]Service number: 14606-85099-KA
Full name: AGU, Kojo
Service number: 92458-37017-EB
Full name: BUCK, Edward
Service number: 21175-12121-TM
Full name: MILES, Taylor H.
Service number: 51033-15973-MC
Full name: CRESPO, Michael
[/quote]
Well, the Rookie's, as it says, is classified. However, his service number ends with JD.
I did some digging in Bungie's super secret archives and found [url=http://img188.imageshack.us/img188/5136/halo3odstrookie.jpg]this![/url]
The doctor is in.