http://www.unofficialbmw.com/images/SparkPlugChart.pdf

e90 and e60 (with N52 engine) are now using iridium spark plugs

"High Performance Iridium Spark Plugs, also referred to as High Power
Iridium Spark Plugs, are incorporated on 2006 model year and later E90, E60
vehicles with the new N52 engine. High Performance Iridium Spark Plugs
incorporate an iridium center electrode (0.6 mm thick), one ground
electrode, and a redesigned insulator. The new spark plug technology allows
for increased service life, improved cold start performance/combustion
characteristics, and a consistent operation throughout the entire service
life. The service life of the new High Performance Iridium Spark Plug is
100,000 miles."

http://www.ngk.de/NGK_CREATES_NEW_IR...RK.2710.0.html