I just installed master (who could resist those spanking new colormaps?? ;) ) and have trouble with the inline backend. nbagg works fine, but when activating %matplotlib inline in the notebook, the MPL object is being returned and REPL'ed, but no graph/image is shown.
I quickly pulled it by doing

pip uninstall matplotlib
Uninstalling matplotlib-1.5.dev1:
  /Users/klay6683/miniconda3/lib/python3.4/site-packages/matplotlib-1.5.dev1-py3.4-macosx-10.5-x86_64.egg
Proceed (y/n)? y
  Successfully uninstalled matplotlib-1.5.dev1
conda install matplotlib

and tried the same again, and it works, so it's apparently not the notebook's fault.

Here's the mini code I ran:

%matplotlib inline
img = np.arange(10000).reshape(100,100)
plt.figure()
plt.imshow(img, cmap='gray')

In other words, MPL master reacts exactly as it would look if I comment out the %matplotlib magic above when running with MPL 1.4.3.