Is there an equivalent distribution that is to SUSE Linux Enterprise Server what CentOS is to Red Hat Enterpise Linux?
OpenSuse is probably the closest to what you're looking for.
There isn't an equivalent to CentOS for SLES and there probably never will be under current Management. AttachMate may think differently, but we have several months before we find out.
More broadly and coming from the other direction, there has been some push in the OpenSUSE community to create a long-term-support version of THAT distribution. The push hasn't gotten that far yet, and seems to be aiming at picking a rev (11.2 seems common) and committing to providing patches for it for much longer than OpenSUSE normally produces patches. This is closer in type to Ubuntu LTS than CentOS, but it's also as close to CentOS as the SUSE ecosystem is likely to get.