Sony's biggest advantage was third party support; third parties threw their biggest handheld games on the PSP because they thought it would eclipse the DS the way the PSX and PS2 beat the N64 and Cube. When that didn't pan out, third parties found themselves out in the cold; many of their fans bought a PSP, and din't want to see third parties move to the DS, while the early third party DS games were underwhelming and aliened people who bought the DS.
By shortchanging the DS early, they got screwed in the long run when the DS took off; and then they repeated their performance on the Wii. After missing the boat twice, third parties do not want to let another Nintendo system slip past them again, and are backing the 3DS early and heavily. Without the advantage of exclusive third party support, there is little chance the PSP2 could compete effectively against the 3DS.
At this point, I imagine that Sony is developing a PSP2, but if third parties are cold to it because they are supporting the 3DS, Sony may decide it's a lost cause and cancel it before announcing it.