I looked at the Valiente commutative diagrams survey
which contains some really nice examples.
What is the state of the matter of making an "official" LaTeX3
commutative diagrams package? -- The thing is that it seems me that one
should have at least a simpler type of commutative diagrams package
developed in connection with Alan Jeffrey arrows package suggestion (which
is at CTAN in the info/ltx3pub directory).
On the packages there mentioned (in Valiente's paper), I can note that
only the Taylor package has the both arrows extending with labels and
arrows extending to meet targets, and only the pstricks package has the
nice curved arrowheads one would expect in a good typesetting, but this
could be easily implemented in any package by using a suitable arrows font.
In addition, a commutaive package must work on two levels: One should be
quickly be able to knock out a correct diagram, and one should be able to
finetune it, just as with other TeX formulas.