Instead of trying to set the axes itself, you can try adjusting the xlim and ylim properties instead:
import matplotlib.pyplot as plt
from numpy import *
xx = arange(0,700,30)
tt = arange(0,100,5)
U = outer(xx,tt)
V = outer(xx**(1/2),tt/4)
plt.subplot(121)
plt.pcolor(tt,xx,U)
plt.colorbar()
#plt.axes([0,600,0,100])
plt.xlim(0,100)
plt.ylim(0,600)
plt.subplot(122)
plt.pcolor(tt,xx,V)
plt.colorbar()
plt.show()