HP’s suggestion that older webOS devices wouldn’t be seeing 2.x updates seems to have been overly cautious: an upgrade to webOS 2.1.0 is now available for the Palm Pre Plus on O2 Germany. According to the support page, Pre Plus owners can punch their serial number into a webOS doctor to find out if they’re eligible; unfortunately right now that means only O2 Germany handset owners get the update.
Still, it does imply that other versions of the Pre Plus on different O2 country carriers will likely see a 2.1.0 update of their own. There’s a lengthy pdf (in German) detailing the changes, and the homebrew community is already tinkering with the ROM to see if they can make something more generic out of it.