I have a script which is not working on Blender 2.69, but it works fine on the latest build from builder.blender.org/download. Perhaps they tweaked something in how rigid body simulations are baked to keyframes.
Should I expect my script to work with the next Blender release?
In other words, do the daily builds have experimental features which will be removed or is it typically just features that are planned to be included with the next release?
Yes they will typically be in the next release.
There's a Variation column on the builder download page, if it's Official, then it's build straight from current master branch.
Features available in these official builds will typically be in the stable releases, although there are rare exceptions when a feature may be disabled (if its not ready) or removed.
It's a new feature, but still under heavy development. It is supposed to be initially released in 2.70. But the current release cycle has been streched a lot already, and people may eagerly wait for other things to be available in a stable release. So the developers may decide to do a release soon. They post-pone the tabs for 2.71, to let them mature, and not throw them in an unfinished state at the users.
If you rely on certain features in an addon for example, and want to ensure compatibility with the next stable release, I would wait for the first Release Candidate (expect 2.70 RC1 at the end of February). If nobody finds bugs in the RC, it will become the stable release. Otherwise there will be another or even a third RC, until all show-stopper problems are solved. Hence, all features of the RC will be in the stable release (functional changes are not allowed in this phase).
