As most of you know, the PlayStation Vita will be able to play PSP games, but the new unit doesn't support the outdated UMD format.
Therefore, Sony Computer Entertainment Japan will have a UMD Passport program in place for the Vita's Japanese release on December 17. Basically, users will register their name for the Passport and then purchase digital copies of UMD titles at a discounted price (and those prices can vary).
As of now, about two weeks before the launch, there are 262 UMD Passport games live. That's not a bad number, but don't forget that there are over 600 PSP games available for download on the Japanese PSN. As noted by Siliconera , it appears that some publishers aren't yet supporting the program. While there are titles from Atlus, SNK Playmore, Tecmo Koei, Nippon Ichi and others, there don't appear to be many (or any) titles from Square Enix, Konami, Namco Bandai and Capcom.
There's every chance those companies will add their support in the near future, but we'll have to wait and see. By the way, Sony has not announced any UMD program for North America or Europe, although they say they're working on one.