OTA Update 5.1.1 (build 9B206) is an official Apple over-the-air update for the iPod touch 3. The update package is 40.06 MB. This is a delta (partial) update that requires 5.1 already installed. This update is currently signed by Apple, so it can be applied from Settings › General › Software Update.

Payload file: 50c3bc3c1c4cda9df1acba0a1e6b18c8943567c3.zip.