Update: Sony's released an official statement, which sadly doesn't provide any real new details on why the program isn't coming over.
"SCEA will not be offering the UMD Passport program. U.S. consumers who already own digital versions of select PSP titles will be able to seamlessly transfer those games to their PS Vita system. What's great for PS Vita owners is that they can enjoy PSP games with upscaled video quality that's ideal for the system's 5-inch OLED screen, along with controls that are optimized for its features such as the dual analog sticks. More than 250 PSP titles (including minis) will be available at the launch of PS Vita, and hundreds of additional titles will be available post-launch."
UMD Passport was a Japanese initiative that allowed UMD owners to pay a fee in exchange for a digital version that would work on their Vita, as Sony’s new handheld does not feature a UMD reader.
The fee proved a point of contention, as there was not a standard UMD-to-digital price, a consequence of publishers being allowed to dictate to dictate pricing. Even Sony didn’t have much consistency, with Patapon 3 and Gran Turismo costing 1,000 yen ($12.89), while Hot Shots Golf 2 cost 500 yen ($6.44).
Many games weren’t even part of the UMD Passport program, severely limiting its overall use.
Still, in general, if the game was available, the price was cheaper than purchasing a brand-new copy of each one, which meant there could ultimately a degree of usefulness for some consumers.
Sony has not yet responded to my request for elaboration on the decision.