A Nexus tablet implies that LG will not be allowed to add its own layer of software on top of Android. The device would reportedly ship this year, possibly as early as "mid-summer or early autumn."
Google has been doing this with smartphones for the last two years, offering a clean version of Android without any software modifications or additions (also called a "pure Google experience"). In January 2010, Google released the Nexus One, manufactured by Taiwan's HTC Corporation. It shipped with Android 2.1 (codenamed Éclair). In December 2010, Google released the Nexus S, manufactured by Samsung Electronics. It shipped with Android 2.3 (codenamed Gingerbread).
The timing for such a device from LG is odd. The Nexus phones were all released at the same time as a new version of Android was introduced, but version 3.0, the first tablet flavor of Google's mobile OS is already out. That being said, it's very possible that this device will arrive with Android 4.0 (codenamed Ice Cream), although the rumor specifically states Android 3.0 (codenamed Honeycomb).
The Nexus phones have not sold particularly well because they are not sold by various carriers. A Nexus tablet, which wouldn't necessarily need carrier support, might be more successful for Google.