Actually, to this one I can answer.

It is a policy (Valid at least through December 2010, as far as I can tell, it may have changed).
It was stated by the director of customer focus group at the operation control center in Atlanta, Cxxxxxxx Rxxxxx (Per rules of FT I did remove the name, though she presented herself and answered plenty of Q&A in the DO).

Her statement was clear:

If a person purchases a ticket on BE to a flight that at the time of purchase is operated on lie-flat equipment, and the flight changes to non lie-flat equipment, delta would compensate for that different. The compensation rate at the time was $500.00 voucher.
She went as far as stating that they have automated the process, agents do not have any discretion about assigning the voucher, and she stated that she can ensure it works.

The reasons I remember it is:
(a) She stated that next year (2011) the pricing may change (voucher assigned), and she did specify amount is likely to be lowered (she even specified the amount, but I do not recall); and
(b) because I approached her and testified that at least in one case (that I've experienced) it did not happened automatically, or at all and all I got were apologies. She was VERY surprised (and I mean- very surprised), wrote my details and the flight details, sent me to send a DM desk email with the details, and followed personally. Then, I got the stated compesation, and apology that it was not auto-posted. The agent that handled that did state that it is documented in the internal manuals.

So, for what it's worth it was the policy at least through Dec/10