I realize that this is an old thread, but someone posted pics on facebook of pens that he engraved with a similar, but slightly less powered 250mw hobby laser. His results looked pretty good.
A few things should be noted as they have been touched in in this thread. First, a rotary attachment is not necessary to engrave one line of text onto a pen barrel. The limited amount of curve is not an issue.
Second, the video that I saw of the laser burning out mentioned that he used the machine no-stop for thirty minutes when it burned out. Using it for that long is contrary to the manufacturer's recommendations. Based on the video, he was clearly running it way too slowly. For our purposes, it would never run for more than a minute at a time to burn a line of text into a pen barrel.
Third, focusing with a larger laser is achieved by raising and lowering the work surface to give you optimal distance. Many people will by or make a tool to allow them to quickly set it to the perfect distance. Instead of raising and lowering the bed, you raise and lower the laser head to maintain the optimal distance (although from the videos that I've seen, many people don't bother).