Not very rare, IMO. When I went and bought mine almost two years ago there were two on the lot. They appear to be less common than some other colors but I would definately not call them rare.
2008 e60 535xi : Alpine White / Natural Brown / Poplar : Auto : Sport : Prem : Nav : Cold Wx : Comfort Access : PDC : Sirius
Aftermarket: K&N Air Filter : Trinity 12W Angel Eyes : Clearbra : Tint : 12mm H&R rear spacers : Michelin Pilot Sport A/S Plus Tires : Debadged : Matte Black Kidney Grille : OEM M5Tech Front Spoiler