A review of the soundtrack at this site has two titles it might be: "Por Una Cabeza" (2:14) and La Violettera (3:34). A page on the Gabrielle Anwar fan site has RealVideo of "The entire 2 1/2 minute tango dance with Donna and LT. COL. FRANK SLADE. (real media, 3,680KB, 148 seconds)". From these two facts, I would guess it's "Por Una Cabeza".

Anybody actually KNOW?

BB,
NightWing

P.S. Haven't seen that movie for ever; gotta rent it one of these days. It's worth it for that scene alone.