How to delete downloaded iOS 6 update?
I had exactly the same problem: OTA will download software updates and they take up space on the device until the user installs them. Then when the next one is released, it takes up more space. You notice them under 'Other' on the capacity graph in iTunes.

To fix I jailbroke and installed the package called "Software Update Killer". It not only prevents future automatic downloads, it deletes the software that's awaiting installation. Notice the shrink in 'Other' next time you sync. For me both 6.0.1 and 6.1 were downloaded and I recovered almost 1.5 GB.