The speculation has been around for a while. The HRC deal, as told to me by some in the know, was 99% certain, but now sits about 95% certain. Word is that a agree to agree deal is in place but they have yet to ink the final signatures.
The reason for the slip to 95% is that HRC may be a little wary that if Cyril doesn't dominate, then they only reason (as perceived from the outside) has to be a problem with the bike.
Now on one hand, this sounds ludicrous as why wouldn't HRC want to employ the services of the best rider given the chance but on the other, I suspect that HRC's rally effort might by a bit of a pet project in Honda and may be operating on a very short leash. If that is the case, they may want to build up performance milestones in a more restrained manner with Sam and Helder. Adding Despres to the mix puts the entire HRC directly in the spotlight and they may not be comfortable with that.
My personal feeling is that HRC have retreated to their secret lair and will debut an entirely new machine and will announce Despres as the rider. I think they will package the two together as a big PR release, perhaps in time for Dos Sertoes or Ruta 40 where Despres will not be up against the might of Coma or Barreda off the bat.
As for Yamaha, I have discounted most of the rumor. I just can't see a rider of Despres' caliber moving to a non-factory effort no matter how well it's performed in recent past. Yamaha has a very good package for a quasi-works team, but Despres, I would expect, would want a more robust structure capable of taking the fight to KTM in a meaningful way, not scrapping for day to day stage wins among a handful of fast guys. He wants to be THE fast guy.
Other than the Yamaha name, what would really be his incentive? He wouldn't be guaranteed any real advantage and if he were beaten handily by Coma, or worse, Faria, then it would simply reinforce the notion that leaving KTM was a mistake. Matching a beating Peterhansel's record on a Yamaha would be sweet, but Despres has no Yamaha background, so I suspect it doesn't mean too much to him. I would think he wants to beat the record and will go with his best, long term chance. Honda may be that only option left for that.