When I was a company commander (ARNG) in the 1990s, I had a PFC eligible for promotion to SPC. Unfortunately, he was not a U.S. citizen, and I could not promote him. I know he planned on obtainig citizenship and reenlisting. Unfortunately, I don't know if he followed through.
Rules may have changed recently with the war on terrorism. I know some service members have taken the oath of citizenship while on active duty. There may be some new rules in effect.