Chas,
Unfortunately, this is something related to some Android devices. They are apparently caching the icons internally on the device. A new tablet and fresh install will result in the "non-beta" icon. I've actually re-installed our app on numerous android tablets, and a good number will retain that old "beta icon. I'm betting it is cached in an android folder or app images. I'll see about tracing it down and seeing if deleting it help.