From the November 11, 2008 BMW diesel live webcast for the press:

Maintenance:
BMW Ultimate Service
Replenishing the Diesel Exhaust Fluid (DEF)
is a no-cost service for the first 4 years or
50,000 miles

You can access a replay of the webcast at BMW Advanced Diesel
The above statement is on the screen at 7:20 into chapter 3. Also during the FAQ section a question was asked about cost of refill and it was stated that no customer would have to worry about that for 4 years or 50,000 miles.

Hope this helps and you work it out. We have a 2010 X5d and when the time comes I fully expect them to cover the cost of DEF