Introductions vs. facelifts:
E65 7er: November 2001 vs. April 2005 (3 years, 5 months)
E85 Z4: October 2002 vs. January 2006 (3 years, 3 months)
E60 5er: July 2003 / March 2007 (3 years, 8 months)
E61 5er: May 2004 / March 2007 (2 years, 10 months)
E83 X3: January 2004 vs October 2006 (2 years, 10 months)
E87 1er: September 2004 vs. March 2007 (2 years, 6 months)
E63 6er: January 2004 vs. September 2007 (3 years, 8 months)
E64 6er: March 2004 vs. September 2007 (3 years, 6 months)
E90 3er: March 2005 vs. September 2007 (2 years, 6 months)
E91 3er: September 2005 vs. September 2007 (2 years)
1er got facelift due new variant introduction: the 3dr E81 model. It would be weird to introduce 3dr model, and then give it a facelift in the next MY.
3er E90 & E91 I guess will be facelifted a bit earlier due new engines & EfficientDynamics. And due stiff competition I guess (mainly C-class & Audi A4). It would be weird not to implement ED features & new engines in best selling model (3er) ASAP , while 1er & 5er already got it.
X3 got facelift due marketing reasons (otherwise there would be too many facelifts in March (5er, 1er), or September 2007 (3er, 6er).
I doubt we will see another second facelifts. Some new features, some new trims & materials in cabin etc usually for new MY. But I doubt design will be any additional exterior & interior design changes.