Currently we're using the svn bundled with Solaris. Blastwave has 1.4.5. And there's always the possibility to compile one's own. But if it's just for the setuuid function it seems moot; I checked the fsfs sources, and all setuuid does is overwrite the file, so you can do that without svnadmin. Having a sharded database might be nice, but not really required as I see it. What else would we gain from 1.5/1.6 on the server?
On the client side, you can use whatever version you like of course.