How "solid" is PyPy?

[miller.paul.w at gmail.com]

I've been looking at PyPy recently, and I see it's reached version 1.0
(and supports language version 2.4).  Given that, I was wondering what
level of backwards-compatibility one can expect from future versions,
i.e. if I run code on, say, a translated stackless PyPy now, what is
the probability that it will run unmodified on PyPy 1.x, 2.x, etc.?

Thanks!