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.