Observed behavior:

>>>>>>os.path.join('/foo/bar', '/baz')
'/baz'

Expected behavior:

should return '/foo/bar/baz' or (suboptimally) raise an exception.

Why:
Observed beahvior violates the minimal surprise principle and does not appear to respond to any particular rationale (by the same token it could return /foo/bar as well)

It does exactly what is documented: "If any component is an absolute path, all previous components (on Windows, including the previous drive letter, if there was one) are thrown away, and joining continues." (http://docs.python.org/dev/py3k/library/os.path#os.path.join)

And the current behaviour is useful; for example, the absolute path of a file name can be computed with os.path.join(os.getcwd(), filename).
This works with both absolute and relative paths.

os.path.join() won't change; you probably should consider another function, like os.path.normpath(a + '/' + b)

Documented it is, and change it may not, but I am still unconvinced.

Anything that joins "should" (IMHO) preserve the leading stuff - this one throws it away - sometimes. Also I do believe a number of other languages take a different stance (I am no pitonista, but took a double take when I noticed this).

I appreciate that you're unconvinced of its usefulness, but you can be assured that some of us do find it a useful and desirable behavior. And Python isn't the only system that works this way, emacs does the same thing when you enter filenames.