I've not come across anything out on the general 'net about converting between the two shader languages. Considering that a good chunk of the demos and books currently use either Cg or HLSL, most of my current work involves porting those demoes across to GLSLang.
I'm particularly interested in the "gotcha's" side of things. That is, I can handle coordinate system conversions and the like simply, but stuff like assumed state coming into the shader or between VP and FP sections etc would be particularly handy to me. I'm not exactly proficient in HLSL (or D3D for that matter), so I'm mostly working with a Monkey See, Monkey Do type of conversion process right now.