As of now...BMW doesn't have the "parts" to fix the issue. The letter says:
IMPORTANT NOTICE
Please note that at the present time, we do not yet have sufficient parts to conduct this recall; however, we will notify you as soon as parts become available. At that time, you should make an appointment with your authorized BMW center in order to have this recall performed on your vehicle.
OTHER INFORMATION
...We will notify you again when parts become available, currently scheduled for March.
This is why I encourage anyone with a 2006 E53 to go to BMW's website and create a profile and provide contact info if you aren't in their system...or not sure if you're in their system. 