[DasIch]

It means that not all features they want to include in 2.0 are in yet but there is enough to warrant a release of some kind.

PyPy is generally so stable that people are using nightlies in production, provided they pass all the tests.

[mikeash]

Actually, it's because it contains some performance regressions. To quote their release notes:

"This release is not a typical beta, in a sense the stability is the same or better than 1.9 and can be used in production. It does however include a few performance regressions documented below that don’t allow us to label is as 2.0 final. (It also contains many performance improvements.)"