That's the M sport bumper vs the standard bumper on the SE. It's body colour plus a grey insert on the 35is and all grey on any model M sport.
It's all the same colour without insert on the non M sport (SE). Been like this since the first e89, at least in the UK.
Black Sapphire BMW Z4 sDrive35i
Mods: HiFi Tweeters Stubby LED Halos & Front Indicators DVDInMotion SmartTop 313 BMW Performance Alloys LCI Repeaters http://goo.gl/PF4nSz