Conclusion

My conclusion is that a new period of 13 baktuns in the Long Count of
the Maya probably begins on 21 December 2012, but that we should not
expect any special phenomena in the sky that are tied to that
beginning.

It seems unlikely to me that the Maya tried to design their Long Count
in such a way that the new period would start on a southern solstice
in the middle of the Milky Way in their distant future, though it is
not impossible.

If the Maya did design their Long Count with that conjunction in mind,
then I do not think that they had enough knowledge to be able to
predict the correct day of the conjunction accurately to better than a
few dozen years, so if any special phenomena could be expected on the
exact day of the conjunction, then they would probably not be visible
on (or not visible just one) 21 December 2012.

I think that the beginning of a new period in the Long Count of the
Maya in 2012 is just as unimportant as the 6000th anniversary of the
Biblical creation date (celebrated in 1996), or the planetary
conjunction of May 2000, or all kinds of past dates for which the end
of the world had been predicted. Someone who did not hear about the
prediction in advance would not have noticed anything special on those
dates, and I predict that December 2012 will be like that as well.

The only effects that can be expected associated with the conjunction
of 21 December 2012 have to do with the attention that people draw to
that conjunction and date. If people expect that unusual things will
happen on a certain date, then on that date they'll behave differently
than usual, and that in itself is already an unusual thing. In this
way they can fulfill their own expectations.