Compatibility with PSN

EDIT: Eurogamer has just put up this article that quotes Sony as saying

"Those who already own any of these titles for their PS3 systems will be able to download this same content to their PS Vita systems for free, and vice versa," Sony said. "There is no need to pay for the same content twice, highlighting the importance of the relationship between PS3 and PS Vita."

However, the feature does not currently extend to all PS3 games - at least, not out of the box.

From this IGN article:

"The rumor suggested that a firmware update would add the feature to all past and future PlayStation 3 games, but Sony has now confirmed that this won't be the case...The compatibility is achieved on the side of the PS3 game, just like normal."

So the streaming functionality has to be written into the PS3 game for it to work, and that is up to each developer.

There is an alternative, though. There are some custom tools that allow any game to be streamed, albeit not without some lag problems or occasional instability.

No sorry, but Sony have said they are aiming to make as many compatible as possible. Heres a list of all the initially compatible games.

I found this FAQ which answers a few questions but I'm not sure if this is the definate answer you were looking for. It says (about half way down):

Q: Is PS Vita compatible with other PlayStation platform games like PS3, PS2, or PSP? Do you have any plan to support other PlayStation platforms in the future?

A: Users can play PSP titles, minis and PSone classics titles which are offered on PS Store. We do no have any plans to make PS Vita compatible with PS3 or PS2 titles at this point.

Q: Is PS Vita compatible with PSP content other than games such as video and comics available on PlayStation Store?

A: SCE plans to make those content available on PS Vita.

Q: Can PS Vita users continue using their save data for PSP games that they have also downloaded from PlayStation Store to PS Vita?

A: We are looking into the opportunity to do so.

The bolded would suggest no, only confirmed titles would be supported however this is really vague and still doesn't completely prove or disprove that the use unsupported games will work. However Sony have already said UMDs aren't supported and if the only way to get the PSP games onto your Vita is through the store then I highly doubt any unofficially supported games will work.

Sorry for not providing any concrete definiative sources I just couldn't find any.