When I run the following python script, everything is fine:
# energy of one unit cell of silicon
#import matplotlib.pyplot as plt
from lammps import lammps
lmp = lammps()
lmp.command("units metal") # A, ps, eV, K, amu
lmp.command("boundary p p p")
lmp.command("atom_style atomic") # only basic attributes
lmp.command("lattice diamond 5.432")
lmp.command("region box block -0.5 0.5 -0.5 0.5 -0.5 0.5 units lattice")
lmp.command("create_box 2 box")
lmp.command("mass * 28.09")
lmp.command("create_atoms 1 region box")
lmp.command("pair_style tersoff")
lmp.command("pair_coeff * * /home/hobler/Downloads/lammps-22Aug12/potentials/SiC.tersoff Si Si")
lmp.command("run 0 pre yes post no")
When I uncomment the second line to import pyplot, however, lammps reports the lattice constant as 5 instead of 5.432 and crashes in the following region command, as it reads all box bounds as 0. This happens with lammps-22Aug12 and matplotlib 0.99-1.2 on CentOS 6.1. It did not happen with lammps-14Jan12 and matplotlib 0.99-1.1 on Ubuntu.
A workaround is to just import matplotlib (and not pyplot). It is not totally uncommon to use pyplot, though, so maybe someone wants to look into this.