I was just comparing images displayed with imshow() with what I get from IRAF's display command and ds9. The latter puts the first axis increasing horizontally and the second axis increasing vertically, as if the image array were addressed as img[ix,iy], where ix and iy are integer x and y coordinates. The axes imshow() produces are labeled as if this is true for it as well, but in fact the image is upside down with respect to what I see with IRAF. Even odder, to see the same display I see in IRAF I have to do imshow(img[::-1,:]), as if I'm reversing the direction of the first axis.
I've tried to look at the source, really, but I feel like I'm *years* away from being enough of a Python hacker to understand matplotlib.