I think I've fixed it.

Running it from the command line (as you suggested) gave more information:

$ solve-field /var/folders/zx/skmp1g7s43xdtwkgzq30gq400000gn/T/fitsU81278
Reading input file 1 of 1: "/var/folders/zx/skmp1g7s43xdtwkgzq30gq400000gn/T/fitsU81278"...
Extracting sources...
simplexy: found 7 sources.
Traceback (most recent call last):
  File "/usr/local/bin/removelines", line 15, in <module>
  File "/usr/local/lib/python/astrometry/util/removelines.py", line 76, in main
    cut=opt.cut, ext=opt.ext)
  File "/usr/local/lib/python/astrometry/util/removelines.py", line 38, in removelines
    T = fits_table(infile, lower=False)
  File "/usr/local/lib/python/astrometry/util/fits.py", line 672, in fits_table
    pf = pyfits.open(dataorfn, memmap=mmap)
AttributeError: 'NoPyfits' object has no attribute 'open'
augment-xylist.c:589:backtick Failed to run command: /usr/local/bin/removelines -e 1 /tmp/tmp.xyls.GBsQG2 /tmp/tmp.removelines.Ck8df4
 ioutils.c:567:run_command_get_outputs Command failed: return value 1

For anyone else with the same problem, I fixed it by installing pyfits for the OSX's preinstalled python using:
sudo easy_install pyfits

It still fails to solve (but it actually makes an effort now). This might be because I need more index files, I'll look into it.

There are a lot of issues with python on OSX, as using homebrew results in the installation of separate copies of python than the preinstalled versions!
Thanks a lot for the help.