It would be an interesting strategy by MS to try and steal some Sony fans over to the 360 by getting them hooked on some franchise on the PSP that won't be coming to PS3. But like everyone else I highly doubt it'll happen.

 more little then the kitty in your sig

Rare is solely the property of Microsoft. Nintendo sold their stake in Rare to Microsoft once Microsoft had purchased majority share in the company. Much of the portable games were a result of backlog most were titles in production before the takeover. As for the two Rare titles on the DS one uses a Nintendo licensed character, and the other is all about market penetration for Microsoft. Viva Pinata is their version of Pokemon.

I would not hold my breath. There is some financial incentive for Microsoft to allow Rare ports on the DS. While there is no financial incentive for them to to lend comfort to their enemy Sony. We have to remember that both Microsoft and Sony are very much working towards the same concept in the gaming space. So there is little room to let bygones be bygones.

It's certainly not impossible, but I doubt it will happen.