Are there any security issues when adding a cmnd_alias for svn to /etc/sudoers?
Well, not if you don't do anything with the alias :)
Assuming you're doing this for a NOPASSWD entry for something, you might want to consider writing shell scripts ( or binaries of some sort ) that perform the specific svn commands you need to be able to run, and then make command aliases for those so that you aren't allowing accidental svn commands that you don't want run.
That's what I would do anyhow -- whitelist to a pretty fine degree of granularity. There aren't any security issues other than the ones that are really obvious that I'm aware of. Someone getting control of the user can run whatever svn commands are allowed. If you go the route of whitelisting via scripts or binaries, someone could replace those scripts or binaries to allow arbitrary execution, but at that point in time your server is rooted anyhow.