Today we have just released Overviewer 0.10.0, with support for Minecraft 1.5 (aka "The Redstone Update")!
Head over to github or our downloads page to get a copy! As usual, stop by IRC, or open a github issue if you run into any problems.
With Minecraft 1.5, the format of texture packs has changed significantly. Previously, textures were all stored in the same terrain.png file. Now, each item and block has its own texture file. You will run into problems if you use Overviewer 0.10.0, but haven't updated Minecraft (or your custom texturepack, if you are using one), so if you don't intend on upgrading Minecraft, you should not upgrade Overviewer.