Out of curiosity, I googled the error message and stumbled upon this FAQ from
gzip.org.  I tried their fixgz program, but the resulting file still gives a CRC
error.  

Simon, maybe it would be worthwhile to see what happens if you

 zcat
_darcs/patches/20080126191939-e23c7-efc06647390321527cc18de0e08c017b568b36ae.gz
> tmp-patch
 gzip tmp-patch
 mv tmp-patch.gz
_darcs/patches/20080126191939-e23c7-efc06647390321527cc18de0e08c017b568b36ae.gz

... and then try the darcs pull.

And here is the FAQ:

========8<==================
gunzip complains about corrupted data or a CRC error

99.9% of the problems with gzip are due to file transfers done in ASCII mode
instead of BINARY mode. In particular, gopher is known to corrupt binary files
by considering them as ASCII. Make sure that your local copy of the file has
exactly the same byte size as the original.

If you have transferred a file in ASCII mode and you no longer have access to
the original, you can try the program fixgz to remove the extra CR (carriage
return) bytes inserted by the transfer. A Windows 9x/NT/2000/ME/XP binary is
here. But there is absolutely no guarantee that this will actually fix your
file. Conclusion: never transfer binary files in ASCII mode. To compile fixgz
and run it, do:

   cc -o fixgz fixgz.c
   fixgz  bad.gz  fixed.gz
   gzip -tv fixed.gz
======8<=======================

Ok, I stop here.  Over to David (or somebody else!)