The whole purpose of the branch detail program is to assign combat support officers to combat arms branches for the first 2-4 years of service. So if you get a branch detail of any type, you are going to start off as some type of combat arms officer. That could be infantry, armor, field artillery, or ADA, for example. There are, of course, no guarantees that you will get MI as your control branch. You could end up getting something like... I don't know... what's the worst thing out there... Signal Corps for example (@Freefalling ;) ).
But there are no guarantees either way. You could request to single-branch MI and end up in the chemical corps for example. "Needs of the Army" and all that. So I recommend you roll the dice on the branch detail. Let the Army know what you want, and see what happens. Whatever you end up being branched, be glad that you have been given the opportunity to serve your country as a commissioned officer, and be the best <insert branch here> 2LT you can be.
By the way, Signal Corps is a very good branch, it was #2 on my combat support picks when I got commissioned. I just tossed that earlier jab in there for the benefit of my good friend Freefalling, who was a signal corps officer back in the day.