I'll be interested to see what OEMs offer this as an upgrade. I'd assume it's easier to upgrade to WM6 than it was to upgrade to WM5 2 years back (at least, it seems to me that it would be...). I predict that some devices with fairly flexible hardware will have some...unofficial... upgrades created. If I had to place bets on who will and won't be getting an update, I'd have to put my money squarely on the Treos
Not getting an update due to the amount of customization Palm has done to WM5. I'd also doubt that the iPaq 69xx series will get an upgrade.
Devices that may get an upgrade? Honestly I have no idea, but I wouldn't be surpised if only new devices will have WM6
Of course, this is just speculation!