Hi Jammerm,
I think that Photo Etching is basically a 'raster process' and the purpose built raster laser machines have lightweight X and Y axis to allow high speeds and accelerations thus allowing the laser beam to possibly travel many miles and complete the job within a reasonable period of time.
Because my machine is basically a wood router and the mass of both the X & Y axis is quite high I think I will just have to content myself with vector use at present.
Having tried a few different materials now I can confirm that Granite, Glass and Slate all engrave well (although I am expecting the glass to shatter anytime soon as I must have put incredible internal stresses into the material).
Tweakie.