It seems that the update brings performance enhancements and fixes, and the big addition allows the homescreen on the Nexus 7 to be rotated into landscape view. According to the Android Development team, this fix is also necessary because:
This obviously confirms that there will be variants to the Nexus 7 hardware, which we've all expected both from the new reports of a 32GB model, and the older reports that there is a space reserved in the Nexus 7 chassis for a rear-facing camera.
The 4.1.2 update has already started rolling out to Nexus 7 users, so just keep checking in for it. The update is 31MB.
**Update** Word has it that the update has also started rolling out to unlocked Samsung Galaxy Nexus phones as well as the Nexus 7.