Do you mean something like this:
>>> from matplotlib import *
>>> plot(xrange(10))
>>> yticks(xrange(10), rotation='vertical')
?
In general, to show any text in matplotlib with a vertical orientation, you can add the keyword rotation='vertical'.
For further options, you can look at help(matplotlib.pyplot.text)
The yticks function plots the ticks on the y axis; I am not sure whether you originally meant this or the ylabel function, but the procedure is alwasy the same, you have to add rotation=’vertical’
Maybe you can also find useful the options ‘verticalalignment’ and ‘horizontalalignment’, which allows you to define how to align the text with respect to the ticks or the other elements.