There are PlayBook apps out there that specify 1.x as minimum OS but you should be aware that:
1) Most PlayBook owners upgrade their OS, so the percentage of devices running 1.x is low,
2) There are lots of deprecated UI components (e.g. qnx.ui.*) from the 1.x days that should not be used,
3) When PlayBook gets upgraded to BB10, it's likely the deprecated UI components will go away.
4) It's my understanding that there are security issues with 1.x that make pirating easy, and some piracy-inclined PlayBook owners have held off on OS upgrades for this reason. You may want to think about whether it's counterproductive to your business to support these folks.
For new PlayBook projects, it seems advisable to:
1) Factor your business logic into its own packages (a general best practice, obviously)
2) Design using qnx.fuse.ui components (and flexible layouts, to support multiple screen sizes, if you plan to have smartphone versions of your PlayBook apps)
3) Get yourself setup with good revision control and bug tracking (also a general best practice), if you intend to fork your codebase to support legacy PlayBook OS alongside BB10 on PlayBook. In the event that BB10 for PlayBook has any major issues that cause folks to delay or completely avoid upgrading, this could be a scenario you want to have at least contingency plans for.
