Well, no.
Incompatibles are pretty much everything new in the SDK except ParseJSON and SetupBehaviorAtTopRow for roListScreen (in 3.1 firmware update released last month).
For example:
all the new Subtitle functions for roScreen
RetainBodyOnError in roURLTransfer
Also for 3.1 firmware, roURLTransfer and roScreen, you must use SetPort instead of setMessagePort - this is forward compatible with newer firmware as well. Only for those components, for the rest, use SetMessagePort.
- Joel