I have a similar prob with handhelds MM. I just can't seem to stay focused on them for very long. That being said, some games that I'd just say you MUST have.
Patapon! Excellent choice my friend >: ) This game is crack >_E
Lumines 1 or 2, both are addictive as hell.... great for more casual type gaming.
Jeanne D' Arc is fantastic!!!!
Castlevania SOTN (I got it off the PSN for like 7 bucks, and the Draken X Chronicles is pretty cheap on the network as well)
Wild Arms (off the network :D $4)
And I agree with the comments on God of War. It was an EXCELLENT game, but I wouldn't pay $40 for it in retrospect :P If you can find it for $30 and under though, go for it.
Also with Crisis Core being more mission based (same with portable ops), it's a pretty good handheld game :) You can get some pretty short sessions in with no prob.
Supposedly a Phantasy Star is coming out for my precious sometime this year. If you've ever played one before you know that it's almost perfect for handhelds and I would totally recommend that as well ^_^
From 0 to KICKASS in .stupid seconds.