Move xticklabels?

I think what Halldor means that it is counterintuitive to

    > have the weekday labels positioned at midnight. The
    > attached example figure shows a week's worth of data, and
    > from a quick glance you'd imagine that the peaks occur at
    > night, while they really are in the afternoon. Aligning
    > the labels' left-hand side with the ticks would help a
    > little, but not nearly as much as having the labels
    > centered between the ticks.

If so, this seems less a problem of tick alignment than tick placing.
One can easily construct ticks for noon.

  import matplotlib.numerix as nx
  from matplotlib.dates import date2num
  dmin = date2num(datetime.datetime(2003,05,10)) # in days

  dateticks = nx.arange(dmin, dmin+7)+0.5 # 7 ticks, at noon
  xticks(dateticks, rotation=-45, ha='left')

It seems to me that you want the tick labels clearly associated with
the ticks, and you should place txhe ticks accordingly, but I can see
how there is room for argument...