Why is it that when I pan something 100% left, I still get a little bit of it in the right channel?  It's subtle so you have to crank the volume to hear it, but this seems really wrong.
 
As a simple test, I just have 1 track panned hard left outputting to master, and only my right monitor on.  I can still hear the track faintly.  Using Xonar X3 build 239 and an RME Babyface.

do you have the monitor hooked up directly to the output of the RME or does it got through a mixer first?

Directly to the RME.  Would love to know if other people can reproduce this.

make sure and check your offset mode to see if that track has inadvertently been set to some value of panning in offset.

Good thought, but nothing going on with offset.  Reproducible across multiple projects, including newly created ones.

Are their any plug-ins in the signal path which may introduce crosstalk?

There is literally nothing going on in this project.  Open new project, add track, set input, hit record, clap in front of the mic a few times.  Pan track hard left (going out to master by default), increase fader volume to max, up RME output volume to max, and I can hear the track coming out of the right channel.  It's quiet, but I can hear it. 

verify your panning laws are still at the default.

I'm set to 0dB center, sin/cos taper, constant power - which I believe is the default.

I'm not sure then.  I'm certain this does not happen to me, especially on every project.  unless someone else has some suggestions, you might want to call tech support.