PGF backend and logarithmic plots


following problem arises when using the pgf backend:

For the plots in my document I would like to use a sans-serif font. Because I want to use the functionality of the Tex-package siunitx (among others) I need to use either the pdf-backend with text.usetex set to True or the pgf-backend. Because I like the idea of being able to use virtually any font with XeLatex I'd prefer using the pgf-backend. However, I cannot seem to be able to convince matplotlib to use a sans-serif font for the tick-labels of my logarithmic axis. The weird thing is, that there is no problem when using linear scales. My (minimal) matplotlibrc looks like this:

backend : pdf : sans-serif
pgf.preamble : \usepackage{sfmath}

An example script to reproduce the error:
#!usr/bin/env python

from pylab import *

y = logspace(-3,9,num=50,base=10.0)

fig = figure(1)
myplt = fig.add_subplot(111)

Has anyone had similar issues or does anyone have a good idea as to what to look into? Any help would be greatly appreciated.

Thanks in advance,