SCEA has said that third party games are not required to be released via digital format.
Quite a few games released post PSPgo launch hit shelves on UMD and have yet to land on PSN. Because each lacks a digital version, Go users cannot play the titles. This is something that is all in the hands of the publisher, according to Sony.
"It's up to individual third parties to decide if and when to publish their UMD releases on PSN," SCEA's Eric Lempel told Joystiq. "Most, if not all, are on board, as this is a revenue opportunity for them."
Wile some companies, like Namco, do not have plans for upcoming games to be released digitally, in the past Sony has required all PS3 games to include Trophies - so it may decide to eventually do the same for Go users.
However, at this time, there are no plans to force the issue.
More through the link.