There are a couple of fuel consumption calculatios in this thread. Here is mine based on what I see as more real world numbers.
Here is what I came up with.
$4.45 for Premium (Lowest price in my area)
$4.95 for Diesel (Lowest price in my area)
35MPG for 335d (What I believe I can get knowing how to drive a diesel for mileage)
25MPG ave for 335i (On the high side but for easier calculation)
$4.45/25 = $0.178 per mile for 335i
$4.95/37 = $0.144 per mile for 335d
$0.178 - $0.144 = $0.034 more expensive per mile to drive 335i
$2000(Cost premium of 335d over 335i)/$0.034 = 58,823 miles to recoup uplift
This is a purely self motivated calculation based on my driving on ave 2k a month for my job, mostly highway miles. I've owned diesels in the past and it is pretty easy to get better than stated mileage. Also, diesels are super easy to mod for mileage and/or power.
I for one am looking forward to 335d. As long as diesel is less than 30% more than premium the uplift will eventually be recouped and there will be ROI.