I've seen a number of dice and rings and such with small lettering "engraved" on them here at Shapeways. What is the smallest or shallowest one can go with such small detail? Is there any certain material that is substantially better suited for this? I'm thinking more along the lines of etched detail like scrimshaw, a snakeskin pattern or the markings on a ruler.
Frosted Ultra Detail is the best material for this. You can go as small as 0.1 mm detail for engraving. See the design guidelines.