From the beta release notes:
FIXED: Previously, the compiler incorrectly allowed synthesis of ivars
belonging to a superclass. This caused crashes or other undefined
behavior when the size of the superclass changed (such as running an
application compiled against an older OS on iPhone OS 3.0.) The fixed
compiler now generates an error when it encounters this condition. If
you have shipped an application to customers, Apple recommends
compiling against the 3.0 SDK and updating your existing application
if the problem exists.
Christopher