I have a RelativeLayout that has a 9-patch background image. The image is a simple speech bubble with a perfectly centered \"arrow\" at the bottom middle.
Make sure you have an mdpi and an hdpi version if you test on different densities. If you put your 9patch in res/drawable/ for instance, it will be treated as an mdpi asset and scaled up automatically on hdpi devices (Nexus S). Because it's an automatic process, the results are not guaranteed.