Just in case you're thinking that Apple employee access was to blame, here are some basic facts: The reader bought the iMac online from the Apple Store and was never in a retail store with it. The reader who sent us these screen shots is not an Apple employee nor does he have access to Apple's RSA Secure ID or VPN. He performed his update at home and not at an Apple retail store.
This kind of update does not normally appear in the wild on Software Update. Prerelease, and specifically "**PRERELEASE**", updates refer to Apple-internal builds distributed to any Apple employee who has access to Apple's VPN. A **PRERELEASE** build is typically seeded to employees 24 to 48 hours before the build goes public via Software Update.
The pre-release weighs in at a whopping 1.19GB. The "About This Mac" screen shots he sent us (see below) note the build as 10D527 – a build that was reportedly released to developers today, only two days after the last build. Builds that come close together typically indicate a public release is imminent. It is not clear why this pre-release showed up on the TUAW reader's iMac.