Nothing is stopping you from using Python 2.x. Writing a new version of a language is never intended to be backwards compatible. Indeed, breaking backwards compatibility is essentially a perquisite because a new version is the opportunity to add all the useful features that couldn't be added before.
That doesn't seem completely true. I could imagine default language syntax changing from one version to another, but still being modifiable via "from __future__ import ..." or "from __past__ import ...", respectively. Unless the bytecodes of the VM are incompatible (which is possible), there isn't any reason py26 code can't talk to py30 code...
