Actually, they recently made a Disagea for the PSP: it bombed pretty bad as far as I know, atleast in the US.
Either way, I know Disagea is PS3-exclusive, supposedly. Nevertheless, I just have a small feeling that Sony will either have to drop alot of cash to keep it exclusive, or will give up rights to it.
Not so much because of Japan (where I believe 1 and 2 did around 250k each), but because of the US and PAL regions (where it does a total of 500k-600k per title).
Back from the dead, I'm afraid.