If you've got a bunch of PSP UMDs (in Japan at any rate) then you'll be able to register them to help you buy discounted PSN versions to play on your shiny new PSP Vita, come December. The system is pretty simple, download an app to your PSP (funny that only now, right at the end of its life, are Sony doing some neat little apps).
Insert your UMD and you get a code you use on PSN for a cheaper version of the PSN game. Hopefully the same scheme will apply to the European and U.S. launches. I'll keep my PSP until the thing dies, so I'm not too worried about those games going over, although playing them upscaled might be good for a couple of titles.
There's a list of around 40 games available, with prices ranging from around £4 to £12, presumably more will follow.