Trine is all but dead. Writing on the company's forums, Frozenbyte's Joel Kinnunen said "It's out of our hands but most likely will not happen. Sorry. (We probably won't comment on this any further.)"
We got a very similar statement from Atlus which told us it's out of the company's hands and probably isn't happening. So if it's out of Atlus' hands and out of Frozenbyte's hands, it would seem the only remaining mitts belong to Microsoft. To be fair though, none of the companies involved would even confirm it was coming, despite the fact it had once been announced by Atlus.
We've asked Microsoft for some more clarification, but don't be surprised if we never get the full story. The important thing to note here is that if you want to play Trine, it's almost certainly going to require a PC or PS3.