CBS is reporting Maj Hasan is a licensed psychiatrist from Maryland.
That means he could have entered post 9/11, incidentally. The Army health corps recruits medical professionals after their med schools and they come automatically at the rank of 0-3 or Captain. E.g., it's feasible he could have been in for as little as 6-7 years.
Also incidentally, if he served as an Army Psychiatrist he would have had < VERY > limited weapons training. I don't know SOP for the Army, but in the Navy/ FMF medical professionals don't even qualify except in special circumstances.