OK, I just posted about this on my blog but thought it will be useful too here, for people using slackware -current.
From the Slackware64 ChangeLog:
Mon Mar 26 01:41:11 UTC 2012
This change gives trouble with some applications. For instance, subversion segfault as soon as it starts. There are patches around for this problem but recompiling and testing glibc is not very funny. Patrick Volkerding will surely fix this soon but if you’re stuck and can not wait, here is a trick: you can use LD_PRELOAD to load the old libc before the new. Updating glibc on Slackware always let essential part of the old lib around, which is handful in this case. The bug in glibc is in the maths lib, so you can run subversion by doing:
$ LD_PRELOAD=/lib64/libm-2.14.1.so svn up
This workaround can save your day!