So while 1.5.X works very well (it's "stable" in that it doesn't crash or work improperly), many pretty important aspects of the design are still likely to change. In terms of the APIs used by boards and libraries, things are still developing. I believe the Arduino Team is actually making a pretty good balance between making the new stuff available versus providing a stable platform that works with a huge number of boards, libraries, and sketches found across the playground and thousands of websites.
I agree with Paul here.
However, in terms of traditional s/w release naming and overall release state,
the 1.5.x code would be in an "alpha" state not a "beta" state.
In traditional s/w release cycle nomenclature "beta" means "feature complete".http://en.wikipedia.org/wiki/Software_release_life_cycle
I know the younger internet generation of whippersnapper devleopers like to push
the envelop of their releases in an attempt to speed things up,
but in true s/w release cycle terminology,
since 1.5.x features are still being added and are morphing around, features are not frozen.
Therefore, no matter how "stable" a given 1.5.x release may be,
feature sets between 1.5.x releases are still changing, and
therefore it is still in an "alpha" release state according to traditional s/w release nomenclature.