I think porting CM9 to the Pre3 would be a worthy goal. While I'm definitely happier with webOS, I do see the possibility that I may have to give in and switch one day. Any real resurrection of webOS via the open source effort is still a long way off, probably more than a year away. By that I mean actual changes that would have daily meaning to users of the platform, not merely the release of Open webOS 1.0.
So, on that front, my TouchPad is covered by CM9, but my only option on my phone is to either buy a new one or stay with webOS. Many may argue that decent android phones are practically free on contract these days, but being off-contract on an unlocked phone is really nice so I have little interest in going back to a contract-subsidized phone.
That all said, I tend to doubt it will happen. There just aren't enough Pre3's out there to make it interesting enough for the CM team to do it. The Pre3 isn't amazing or unique enough for Android people to say "oh, if only we could get our hands on that amazing Pre3!!!". The only value in an android port for Pre3 is for those who already have Pre3's and would like to switch.