Riviera: The Promised Land (as I learned to my surprise) was part of a series of games. Guess what? Yggdra Union is part 2 of that series, and the US will see it's release for the PSP on September 16th 2008. The game will boast a few extra features from the original GBA game including new characters, missions and a dual-language voiceover track so those who hate American voice acting can listen to the Japanese instead.
The game has not been confirmed for Europe.