This has ticked me off for a bit, and I finally decided to find out why. Tab completion in zsh was returning the following every time I tried to use it, after upgrading to subversion 1.5:
_arguments:comparguments:303: invalid argument: ARG
Looks like the reason is that the subversion developers changed "arg" to "ARG". So when I googled I found this helpful post:
ZSH Tab Completion Fix for Subversion 1.5 which basically points to this new _subversion functions file to be plopped into the zsh functions directory (/usr/share/zsh/4.3.4/functions; change the version to suit of course -- the directory may also be slightly different on Linux vs OS X).
Now tab completion works properly and life is good. =)