Key features which will be unavailable to Android apps running under the compatibility layer on the PlayBook and future BlackBerry devices include Android's famed battery-sucking Live Wallpaper, SIP and SIP VoIP, anything built using the Native Development Kit, apps containing only App Widgets, and apps containing more than one activity tied to the Launcher.
In addition, any packages which rely on Google Maps, in-app billing services, Android's text-to-speech engine, or the cloud-to-device messaging system will all be rendered unusable under the company's runtime system.
So basically, figuring out what will run and what won't will be a huge nightmare, so much so that RIM might as well have not bothered adding this feature in the first place. It's certainly not going to allow PlayBook owners to make use of the Android Market (yes, there were people expecting this).
I think that this is all pretty moot now anyway. It seems that the PlayBook is on the way out. While RIM says that it is still committed to the PlayBook, retail partners are desperate to get rid of stock, offering $200 off the price of the device ($100 coupon and $100 rebate), making the 16GB WiFi version $299. That sort of deep discounting is not a good sign.