Number one: I'm not in the military
Number two: This is only what I have found, not what may be the truth
That being said, the recruiter can't promise you a job, but after going through MEPS and taking the ASVAB and other tests, you can get a guaranteed apptitude area, and then get assigned a job after basic training.
So basically your abilities help you get the job. If your going ROTC then you'll most likely go commisioned after finishing your degree.
Here's some info on Intelligence as an officer
and some on Enlisted (if you decide that route)
On a second note, if you go enlisted I know that you'll be taking ASVAB. I found a few tips to increase your chances to get into one of the four fields that you want. If you scroll down on this page a little bit you'll find some info on the areas that are focused on for different career fields http://usmilitary.about.com/cs/joini...svababcs_4.htm
. BY studying for and focusing on certain areas of the ASVAB you increase your chances of getting into the apptitude that your desired career is in.
There's quite a bit of info on this site and this is where I've gotten my answers from, so if there wrong, sorry for the misdirection. I'm waiting to see how a current job prospect pans out, if not, I will be joining the Air Force so I've been looking for some answers too. Hope some of this info helps you.