By the way, for now every release of V8 is using a different SONAME, because upstream cannot guarantee binary compatibility even across minor releases. Because of this, all applications using V8 have to be recompiled after updating the shared library.
I am looking for a few solutions to this problem. First, I think it is not realistic to make a few releases share the same SONAME without upstream cooperation (for a few reasons read Diego's Your symbol table is not your ABI). However, generally the releases seem to be compatible: http://linuxtesting.org/upstream-tracker/versions/v8.html. Also, because V8 is now independent from the applications (i.e. not bundled with them), you should not need to update it unless necessary.
It is not obvious how well this is going to work, so any feedback is welcome, especially if you hit bugs.