According to David Wood (a leading Symbian Foundation),there will be a large break in binary compatibility with Symbian^4 (emphasis by editor). Such a step is not taken lightly, but it seems to be the emerging view of the Symbian community that the large benefits of this break will outweigh the undeniable drawbacks.

The big picture here is that a change in UI idiom and development environment will result in very significantly improved productivity, and in turn in a huge range of impressive and attractive applications.

So, for a developer who currently does S60v5 apps: what is the latest release of Symbian which will be able to run them?

The answer was that these applications should run fine on devices built with Symbian^3. Engine level components of applications (which don’t use any S60 Avkon APIs) should run fine on later devices.

More to read over here !